Support sellection ranges LSP API

[matklad]

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

The TL;DR of the feature is that the server informs the editor about the ranges of syntax constructs at the cursor position (eg, the expression, statement, function, etc at cursor), which the editor than uses to provide smart "extend selection" action which is helfpul to select syntactically self-contained fragment.

This is a relatively important feature to support, as it unlocks powerful editing capabilities.
It also should be relatively easy to add support for, as it only needs information about syntax.

[SuperAuguste]

Huh, I've never seen this before - looks awesome! Let me know if you'd like to take a jab at it, otherwise someone'll get to it eventually :)

[matklad]

I am not very likely to look into this super-soon, but will probably get to this eventually if no one beats me to it.

Any pointers to the source code (eg, where to add support for new LSP request, where to add tests, how to get a syntax tree) would be appreciated!

[SuperAuguste]

New requests can be added here - just model your new function based on the others present. Here's an example showing how to get a DocumentStore.Handle (handle.tree will get you an AST). You can plop new tests in here - just model them after existing ones.