Simplify registering APIs in ClassDB and use macros to register classes instead of functions in example.

[Daylily-Zeleen]

Refer to this comment.

And I found that in godot source, register of all classes are using GDREGISTER_XXX_CLASS(T) instead of using ClassDB::register_xxx_class<T>(), so I think we should change the example to encourage users to register classes by macros.

But this pr will change apis of ClassDB significantly and violate programming principles, so this pr is just suggestive and need more discussion.

This is godot-cpp version of godot/pull/81547, but not depends on it.

[dsnopek]

I'm 100% in favor of switching to the GDREGISTER_XXX_CLASS() macros in tests and examples!

But for simplifying the ClassDB::register_xxx_class() functions, I think we should only make that change if Godot makes the same change.