feat: allow finer-grained control of insertText

[wkillerud]

See the documentation for server-only settings for the details.

For #230

TODO:

• Regression test Vue, Astro and Svelte in VS Code
  • User-authored styles
  • Sass built-ins
• Test Vue, Astro and Svelte in Sublime Text
  • User-authored styles
  • Sass built-ins
• Write automated tests for Astro and Svelte in language-services, they weren't covered it seems

[wkillerud]

Before committing to all these settings: the spec allows for a different type of completion item that does a text edit with specified ranges.

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_completion

Right now we use the simple model where

the client should filter against what the user has already typed using the word boundary rules of the language

I know the word boundary is what's caused some of the issues with . and $ in VS Code.

The second, while seemingly more complex, could well end up simpler for the server implementation, but certainly for language clients who won't have to deal with all these settings.

I'll leave this draft PR open, but I want to restart and try this other model with text edits before I continue down this path.

[wkillerud]

#232 is much more promising and future-proof