LSP Jump to Definition for LOCAL bindings

[ChrisPenner]

Overview

Adds LSP support for "jump to definition" for local bindings.

After the changes for local-var type info this was absolutely trivial to add :)

Notes:

• this uses the same implementation for 'jump to definition" "jump to declaration" and "jump to implementation" when invoked on locally bound var references.
• Types, and jumping to things in your codebase aren't implemented and aren't likely to be implemented unless VSCode changes its "prefetch" behaviour :'(

Implementation notes

• Use the binding location information collected during typechecking
• When invoked, use the location of the cursor to look up the relevant symbol, map that to its binding site, then jump there.

Test coverage

• None yet, a bit annoying to write, but could try to dream some up; will probably wait until someone notices it broke to go through the effort since it's not trivial to do, nor is it a mission-critical feature.