fix: use textEdit for completion items insertion

[wkillerud]

The edge cases in different editors with regards to word boundaries has been too high (even for syntaxes in the same editor).

Since perhaps not all clients support it, adopting this without checking for clientCapabilities would be a breaking change. We keep the existing behavior for backwards compatibility. Nevermind, I'm looking at it wrong. From 3.16 is the new InsertReplaceEdit. textEdit is as old as the spec.

TODO:

• Add textEdit variant of all completion items.
• Use clientCapabilities to decide whether or not snippets should be used (while we're at it)
• Manual testing in VS Code
• Manual testing in Sublime Text
• Manual testing Helix
• Update the automated tests with new assertions
• Update documentation
• Call out now unused settings in release notes

Replaces #231

For #230