Add improved code intel support for Starlark
Created by: olafurpg
First steps towards https://github.com/sourcegraph/sourcegraph/issues/37761
Adds squirrel navigation for:
- local variables
- string literals in
load(...)statements
Starlark has a more features that we need to implement eventually, but this is a decent milestone towards that goal.
Test plan
See new Squirrel tests.