Add a handler for "Go To Declaration"

[michaelpj]

At the moment we mostly just use "go to definition", but LSP also has "go to declaration". I think the concept of a declaration does actually make sense in Haskell, namely:

1. For entities that have a type/kind signature, go to declaration could go to the signature (which can be in a different location)
2. For typeclass methods, go to declaration could go to the declaration in the class, and go to definition could go to the definitions in the instances
3. Anything else?