Implement semantic tokens

[jakebailey]

Fixes #1894

I am a little scared of this one. VS Code seems to ask for doc highlights, semantic tokens, diags, all in quick succession. All three of those require checkers, which means we are likely creating 3 checkers to handle these. 4 if we merge inlay hints.

[DanielRosenwasser]

VS Code seems to ask for doc highlights, semantic tokens, diags, all in quick succession

Wasn't the same true for document highlights and semantic tokens?

Does it feel reasonable in a larger codebase?

[DanielRosenwasser]

Ah, but we didn't make more checkers.

[jakebailey]

Drafting this for now, until we can improve the checker situation IMO