Add a handler for "Go To Declaration"

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?

Begin by reading the existing “go to definition” handling and the LSP specification for “go to declaration,” then determine how Haskell signatures and typeclass methods should map to declarations. Done means the behavior is agreed for the listed cases and any additional cases, implemented, and covered by tests.

Written by the indexing model from the issue text.