provides `Diagnostic` for unused variable

[dannypsnl]

This PR creates three things for "unused variable"

1. diagnostic hints for it
2. ignore variables that prefix with _, e.g. _xxx
3. quick fix that fixes it with prepending _ to variable

Notice that the quick fix needs the client's ability

@jeapostrophe do you know who can update magic-racket to enable textDocument/codeAction?

demo diagnostic

demo code action

[jeapostrophe]

Cool!

[dannypsnl]

@jeapostrophe This is ready by the way.

[jeapostrophe]

Excellent!