Derive `GodotClass` for traits

similar to #334, a possible way to add restricted inheritance would be to allow traits to derive `GodotClass`. This would turn that trait into an abstract class. Then later we could allow people to inherit from this base class by implementing the trait.

i haven't thought out many details of this, but it has the advantage over doing it for enums that we dont need to enumerate all possible children immediately. it's also more similar in both godot and rust.

we could possibly integrate this with traits in gdscript when they're added: https://github.com/godotengine/godot-proposals/issues/6416

We'd probably need to make the trait a subtrait of `Inherits`.