Support internal abstract classes

[Naros]

Godot version

4.4.1

godot-cpp version

4.4.1

System information

Fedora 42

Issue description

Currently we can define GDREGISTER_INTERNAL_CLASS or use ClassDB directly but there is no exposed way to register classes that are both abstract and internal in Godot, which would be super useful for GDExtensions.

Steps to reproduce

N/A

Minimal reproduction project

N/A

[dsnopek]

What's your use case for making a class both internal and abstract?

Internal classes can't be created by anything except the GDExtension itself, and the Godot editor in some select cases; they can't be created in GDScript. Godot will go out of it's way to hide internal classes.

Abstract classes can't be created by anything except the GDExtension itself, period. However, they aren't hidden in any way.

I'm not sure what the properties of a internal and abstract class would be? I feel like its "internalness" would end up overriding most of its "abstractness", and it's just an internal class that the editor can't create (in the handful of cases where the editor would be able to create an internal class, for example, as an editor plugins).