Add support for registering virtual and abstract classes

[rburing]

Core PR: godotengine/godot#66979.

Bugsquad edit: Fixes #872

[akien-mga]

Thanks!

[aaronfranke]

It's working if the subclasses are instanced from GDScript using .new(), but for node types, they do not show up in the Create New Node dialog, and they do not work if I manually edit the .tscn with the type.

Here is with ClassDB::register_class<CollisionObject1D>();

Now, if I change it to ClassDB::register_abstract_class<CollisionObject1D>();, StaticBody1D etc disappear from the menu and it also doesn't work if I set type="StaticBody1D" in the .tscn file. But add_child(StaticBody1D.new()) works.