Allow registering "runtime classes" in modules (not just GDExtension)

[dsnopek]

PR #82554 added the ability to register "runtime classes" from GDExtension.

This PR allow registering them from modules too!

It works just like the GDExtension version where it generates a fake ObjectGDExtension struct for the placeholder instances, but in this case it's generating it for a native class rather than a GDExtension class.

I personally think this is worth implementing, because:

1. We want Godot and godot-cpp to provide the same API as much as possible. In fact, we usually tell contributors wanting to add something to godot-cpp that it needs to be supported in Godot before we can add it to godot-cpp. This is going a little bit in reverse :-) but it allows us to maintain that goal.
2. It makes it possible for a GDExtension with "runtime classes" to optionally be compiled as a module. This is somewhat related to the first point, because this is part of the reason we want godot-cpp and Godot to have the same API, but I think this is worth calling out separately. Being able to easily compile a GDExtension as a module could be especially useful if porting a game to a platform that doesn't support GDExtension.
3. Some people do code their gameplay logic in a module, and this makes that more convenient. While it's a very, very small minority of people who do this, there are some people who code their gameplay in a custom module. This will make that more convenient in the same way it makes coding gameplay logic in a GDExtension more convenient.
4. The implementation is pretty small. It's not that much additional code to support this, so, why not :-)

[akien-mga]

Makes sense to me to expose this to modules.

[akien-mga]

Thanks!