Add semantic coloring for other #langs

[mlavrent]

I'd like to add syntax highlighting for other #langs besides Racket. I do really enjoy the current Racket colorings (more so than the DrRacket ones!) so the proposal would be to:

• continue using the current colorings for #lang racket and all other #lang racket/<*>
• use semantic coloring from racket-langserver (this would need to be added) for other #langs.

Let me know if you think this is a good idea or not!

[Eugleo]

It is! I think it would even be worth to refactor the current regexes, because there are some minor bugs with Racket highlighting. It would also be great if we were able to combine the current regex-based solution with the dynamic highlighting the lsp can provide, even for Racket — we could then have proper quote/unquote behaviour, for example.