[Suggestion] Allow RequiredArgumentBuilder to be extended

[willkroboth]

This PR makes RequiredArgumentBuilder's constructor protected, which makes it possible to create a class that extends RequiredArgumentBuilder.

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Currently, while RequiredArgumentBuilder is a public, non-final class, its only constructor is private:

brigadier/src/main/java/com/mojang/brigadier/builder/RequiredArgumentBuilder.java

Lines 11 to 25 in b92c420

	public class RequiredArgumentBuilder<S, T> extends ArgumentBuilder<S, RequiredArgumentBuilder<S, T>> {
	private final String name;
	private final ArgumentType<T> type;
	private SuggestionProvider<S> suggestionsProvider = null;
	private RequiredArgumentBuilder(final String name, final ArgumentType<T> type) {
	this.name = name;
	this.type = type;
	}
	public static <S, T> RequiredArgumentBuilder<S, T> argument(final String name, final ArgumentType<T> type) {
	return new RequiredArgumentBuilder<>(name, type);
	}
	public RequiredArgumentBuilder<S, T> suggests(final SuggestionProvider<S> provider) {

This makes it impossible to extend RequiredArgumentBuilder, since there is no valid accessible signature for the super call in a subclass.

On the contrary, LiteralArgumentBuilder's constructor is protected:

brigadier/src/main/java/com/mojang/brigadier/builder/LiteralArgumentBuilder.java

Lines 9 to 21 in b92c420

	public class LiteralArgumentBuilder<S> extends ArgumentBuilder<S, LiteralArgumentBuilder<S>> {
	private final String literal;
	protected LiteralArgumentBuilder(final String literal) {
	this.literal = literal;
	}
	public static <S> LiteralArgumentBuilder<S> literal(final String name) {
	return new LiteralArgumentBuilder<>(name);
	}
	@Override
	protected LiteralArgumentBuilder<S> getThis() {

Subclasses are allowed to access protected members of their superclass, so it is possible to extend LiteralArgumentBuilder.

Since RequiredArgumentBuilder and LiteralArgumentBuilder are not final, it seems to me that were intended to be extendable.

[willkroboth]

I realized this is exactly the same PR as #122. I don't think I'll close this though, since double the PRs means double the chance this gets noticed by Mojang :P.