[feat] syntactic semantic tokens

[MangoIV]

Use the GHC AST and lsp semantic tokens to convince the language server to give highlighting even without any editor highlighting plugins.

Fixes #4501

• check that this works on all ghc versions
• check which AST nodes I might have overlooked that are important.

[soulomoon]

Some keypoints beyond the tokens retrieval:

1. UTF8 vs UTF16 differences for the token range between haskell and LSP spec. see focusTokenAt.
2. Consider position mappings so we have correct highlighting even the file could not even parse, see toCurrentRange.
3. we need to consider delta between each result, see semanticTokensFullDelta. This is to minimize the bandwith between LSP server and clients.

Post it here as a reminder.

[MangoIV]

Thank you. I will go through all of these.

[MangoIV]

@soulomoon I'm currently struggling a bit with the tests - the ASTs are different enough that making the syntax highlighting consistent between ghc versions is probably a big pain. Should I introduce more goldens for all supported ghc versions?