Investigate whether we can use Engine.register_singleton() to allow mods to add global classes.

we are creating dynamic zips for mod hooks already.

this would remove the need for an `override.cfg` and give us more control over the file in general to reduce crashes if the classes changed for example

either
- [`Engine.register_singleton()`](https://docs.godotengine.org/en/stable/classes/class_engine.html#class-engine-method-register-singleton)
- by loading a resource pack with a modified class cache