Results 5 comments of PB

Added a ReferenceProvider based on mix xref.

I think I am done... for now. Please let me know if there is anything to add/fix.

I've added a WorkspaceSymbolProvider. Now it's sort of complete. Please review. I stop working on it, so no more commits in this PR :)

I've added ReferenceProvider. It uses "mix xref callers ..." and can scan references of a module or function. mix xref even supports arity, but arity determination is sort of tricky...