First attempt to include the semantic highlight feature, introduced in specification 3.16.0.

[jlahoda]

This is a crude attempt to include semantic highlights introduced in LSP spec 3.16.0: https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_semanticTokens

It is mostly copied src/message_handler.cc#emitSemanticHighlight, with some tweaks. Feedback is welcome!

[MaskRay]

Thanks! I guess the specification is probably a bit unfortunate, since the payload may be much larger than the ccls extension...

[FelipeLema]

I've been using this branch today and it looks like it works all right (at least, for starters… I didn't see any obvious errors or problems)

@MaskRay is there any documentation of how to handle the missing TODOs? (I didn't seem to run to any of these errors, btw)