Add Roslyn-inspired model for settings/options

Nothing uses it yet but it'll be used by MonoDevelop.MSBuild,
and at some point used by the XML formatter and XML editor.

Core/IsExternalInit.cs

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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Runtime.CompilerServices;
+
+// Declare this to get init properties. See https://github.com/dotnet/roslyn/issues/45510#issuecomment-694977239
+internal static class IsExternalInit { }

Core/Options/IOptionsReader.cs

@@ -0,0 +1,9 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace MonoDevelop.Xml.Options;
+
+public interface IOptionsReader
+{
+	bool TryGetOption<T> (Option<T> option, out T value);
+}

Core/Options/OptionReaderExtensions.cs

@@ -0,0 +1,15 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace MonoDevelop.Xml.Options;
+
+public static class OptionReaderExtensions
+{
+	public static T GetOption<T>(this IOptionsReader options, Option<T> option)
+	{
+		if (options.TryGetOption<T> (option, out T value)) {
+			return value;
+		}
+		return option.DefaultValue;
+	}
+}

Core/Options/Option`1.cs

@@ -0,0 +1,50 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+
+namespace MonoDevelop.Xml.Options;
+
+/// <summary>
+/// Defines an option that may affect formatter, editor, analyzer or code fix behavior.
+/// Some of these are read from .editorconfig, and others may be mapped to equivalent settings
+/// of the host IDE.
+/// </summary>
+public class Option<T>
+{
+	public Option(string name, T defaultValue, bool isEditorConfigOption)
+	{
+		Name = name;
+		DefaultValue = defaultValue;
+		IsEditorConfigOption = isEditorConfigOption;
+	}
+
+	public Option(string name, T value, EditorConfigSerializer<T>? serializer = null) : this(name, value, true)
+	{
+		Serializer = serializer;
+	}
+
+	/// <summary>
+	/// A unique name for the option. If this is an editorconfig option, this will be used as the name
+	/// in .editorconfig.
+	/// </summary>
+	public string Name { get; }
+
+	/// <summary>
+	/// The value to use for this option when no setting is found in EditorConfig or
+	/// in the host.
+	/// </summary>
+	public T DefaultValue { get; }
+
+	/// <summary>
+	/// Whether this option will be read from .editorconfig.
+	/// </summary>
+	public bool IsEditorConfigOption { get; }
+
+	/// <summary>
+	/// Optionally override the EditorConfig serialization behavior
+	/// </summary>
+	public EditorConfigSerializer<T>? Serializer { get; }
+}
+
+public record EditorConfigSerializer<T> (Func<string, T> Deserialize, Func<T, string> Serialize);

Core/Options/TextFormattingOptionValues.cs

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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace MonoDevelop.Xml.Options;
+
+// based on https://github.com/dotnet/roslyn/blob/df4ae6b81013ac45367372176b9c3135a35a7e3c/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/LineFormattingOptions.cs
+/// <summary>
+/// Captures common text formatting options values from an <see cref="IOptionsReader"/>
+/// so that they may be accessed more efficiently.
+/// </summary>
+public sealed record class TextFormattingOptionValues ()
+{
+	public readonly TextFormattingOptionValues Default = new ();
+
+	public bool UseTabs { get; init; } = false;
+	public int TabSize { get; init; } = 4;
+	public int IndentSize { get; init; } = 4;
+	public string NewLine { get; init; } = Environment.NewLine;
+	public bool TrimTrailingWhitespace { get; init; } = false;
+
+	public TextFormattingOptionValues (IOptionsReader options)
+	   : this ()
+	{
+		UseTabs = options.GetOption (TextFormattingOptions.UseTabs);
+		TabSize = options.GetOption (TextFormattingOptions.TabSize);
+		IndentSize = options.GetOption (TextFormattingOptions.IndentSize);
+		NewLine = options.GetOption (TextFormattingOptions.NewLine);
+	}
+}

Core/Options/TextFormattingOptions.cs

@@ -0,0 +1,46 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace MonoDevelop.Xml.Options;
+
+// based on https://github.com/dotnet/roslyn/blob/199c241cef61d94e25fcfd0f6bcaa91faa35d515/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/FormattingOptions2.cs#L23
+/// <summary>
+/// Options that control text formatting. Accessing these multiple times may be done more efficiently using <see cref="TextFormattingOptionValues"/>.
+/// </summary>
+public class TextFormattingOptions
+{
+	public static readonly Option<bool> UseTabs = new ("indent_style", false,
+		new EditorConfigSerializer<bool> (str => str == "tab", value => value ? "tab" : "space")
+	);
+
+	public static readonly Option<int> TabSize = new ("tab_size", 4, true);
+
+	public static readonly Option<int> IndentSize = new ("indent_size", 4, true);
+
+	public static readonly Option<string> NewLine = new ("end_of_line", Environment.NewLine, new EditorConfigSerializer<string> (
+		str => str switch {
+			"lf" => "\n",
+			"cr" => "\r",
+			"crlf" => "\r\n",
+			_ => Environment.NewLine
+		},
+		value => value switch {
+			"\n" => "lf",
+			"\r" => "cr",
+			"\r\n" => "crlf",
+			_ => "unset"
+		}));
+
+
+	public static readonly Option<bool> InsertFinalNewline = new ("insert_final_newline", true, true);
+
+	public static readonly Option<bool> TrimTrailingWhitespace = new ("trim_trailing_whitespace", true, true);
+
+	public static readonly Option<int?> MaxLineLength = new ("max_line_length", null, new EditorConfigSerializer<int?> (
+		str => str != "off" && int.TryParse (str, out var val) && val > 0 ? val : null,
+		val => val.HasValue && val.Value > 0 ? val.Value.ToString () : "off"
+		));
+}

Core/Options/XmlFormattingOptions.cs

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+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace MonoDevelop.Xml.Options;
+
+/// <summary>
+/// Options that control XML formatting
+/// </summary>
+public static class XmlFormattingOptions
+{
+	public static readonly Option<bool> OmitXmlDeclaration = new ("xml_omit_declaration", false, true);
+	public static readonly Option<bool> IndentContent = new ("xml_indent_content", true, true);
+
+	public static readonly Option<bool> AttributesOnNewLine = new ("xml_attributes_on_new_line", false, true);
+	public static readonly Option<int> MaxAttributesPerLine = new ("xml_max_attributes_per_line", 10, true);
+
+	public static readonly Option<bool> AlignAttributes = new ("xml_align_attributes", false, true);
+	public static readonly Option<bool> AlignAttributeValues = new ("xml_align_attribute_values", false, true);
+	public static readonly Option<bool> WrapAttributes = new ("xml_wrap_attributes", false, true);
+	public static readonly Option<int> SpacesBeforeAssignment = new ("xml_spaces_before_assignment", 0, true);
+	public static readonly Option<int> SpacesAfterAssignment = new ("xml_spaces_after_assignment", 0, true);
+
+	public static readonly Option<char> QuoteChar = new ("xml_quote_style", '"', new EditorConfigSerializer<char> (
+		str => str == "single" ? '\'' : '"',
+		val => val == '\'' ? "single" : "double"
+		));
+
+	public static readonly Option<int> EmptyLinesBeforeStart = new ("xml_empty_lines_before_start", 0, true);
+	public static readonly Option<int> EmptyLinesAfterStart = new ("xml_empty_lines_after_start", 0, true);
+	public static readonly Option<int> EmptyLinesBeforeEnd = new ("xml_empty_lines_before_end", 0, true);
+	public static readonly Option<int> EmptyLinesAfterEnd = new ("xml_empty_lines_after_end", 0, true);
+}

Editor/Options/XmlEditorOptions.cs

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+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace MonoDevelop.Xml.Options;
+
+/// <summary>
+/// Options that control the behavior of the XML editor
+/// </summary>
+public static class XmlEditorOptions
+{
+	public static Option<bool> AutoInsertClosingTag = new ("xml_auto_insert_closing_tag", true, false);
+	public static Option<bool> AutoInsertAttributeValue = new ("xml_auto_insert_attribute_value", true, false);
+}