Detect conflicts with reserved/existing identifiers

[Bromeon]

Sometimes, users accidentally choose a name that conflicts with a built-in Godot name, or a registered symbol in a base class. They often cause non-obvious errors at runtime.

We should see if we can turn them into compile-time or startup-time errors. Startup-time is OK, if we don't want macro-hardcoded logic, so it would be forward-compatible to builders and more dynamic registrations. Either way, this would be a big improvement over the status quo.

These conflicts come in many forms:

• Classes (IIRC already fails to register, not sure about error message)

• #[func] definitions

  • virtual method names (e.g. _ready)
  • other functions in the base

• #[var] / #[export] properties

  • property of same name exists in the base class
  • or the getters/setters exist as functions

• #[signal]

• #[constant]