Metals: Hover provider

[gabro]

For names that have a matching symbol this should be easy.

@xeno-by / @olafurpg how far away are we from having type-at-point support in scalameta? 😇

[olafurpg]

I think Denotation.signature will do a decent job for type-at-point. It won't resolve the type as seen from the call-site (map in List(1).map(...) won't take into account List[Int] prefix) but I think the raw signature is a lot better than nothing at all.

[gabro]

Speaking of Denotation.signature, as suggested in #2, textDocument/signatureHelp looks within reach too!

(It would also be great to have scaladoc for methods and classes in semanticdb, but I've no idea of the difficulty)

[olafurpg]

Uncompiled scaladocs (no resolution of [[Name]]) are probably quite easy to support, I have an implementation that maps docstring tokens to definitions that we can probably run inside semanticdb-scalac.