Support semantic highlights

[4e554c4c]

Agda is a very difficult language to parse.

Because of this, many other highlighting methods (such as textmate, or treesitter grammars) are not possible.

If the agda-language server supported semantic tokens, then many editors would be able to highlight agda code much better.

[L-TChen]

Ping @banacorn

[nkaretnikov]

This was discussed at the most recent Agda dev meeting as something to look into.

I'd like to take a look at this once I'm done with the initial work of navigating/documenting the VSCode extension and the language server.