Add hovers for Scala sources

[olafurpg]

Currently, hovers are not working for Scala sources because the generated SemanticDB document is missing SymbolInformation nodes. The symbol information is missing because the Metals mtags module explicitly disables symbols here
https://github.com/scalameta/metals/blob/75355b207cf89943408d0726797b8a37f3aa1278/mtags/src/main/scala-2/scala/meta/internal/pc/SemanticdbTextDocumentProvider.scala#L36

We should contribute a PR upstream to make this behavior configurable.

[olafurpg]

Pending PR scalameta/metals#3018

[olafurpg]

Fixed in #280