ClassDB macros and class registration

### Godot version

4.3 beta 1

### godot-cpp version

master

### System information

Windows 10 and Linux Mint

### Issue description

I recently updated the godot-cpp for my project and noticed now that abstract classes require `register_abstract_class<>()`. Yet, if we want an internal abstract class we can't with any of the public functions. Would there be a problem if a public function could be provided allowing any possible combination? Or in other words, make the `_register_class()` template function public.

While taking a look at the *class_db.hpp* file I noticed there are a few macros meant for class registration, however none of them takes into account a project that doesn't use the godot namespace.

### Steps to reproduce

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### Minimal reproduction project

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