Rewrite gdscript-imenu to provide GDScript-specific tables

While we have imenu support, it's based on the built-in Python package and not tailored to GDScript, creating tables that are useful but that also have incorrectly sorted or formatted entries at times. For instance, signals get labeled as "Structs".

Tasks:

• Correctly label functions, variables, and inner classes.
• Add support for optional type hints, at least for functions. We need a regexp that captures -> type. It'd be nice to have for variables too but with inferred types we're a bit limited.