Allow GDExtension to access core singletons in INITIALIZATION_LEVEL_SERVERS

[dsnopek]

There are long-standing tricky problems with GDExtension accessing the core singletons, due to the order that they are registered and created.

This PR moves the execution of GDExtension's INITALIZATION_LEVEL_SERVERS until after the core singletons are registered.

This does create a difference between modules and GDExtensions: modules have INITIALIZATION_LEVEL_SERVERS run before the singletons are registered. However, modules don't actually need the singletons be registered, because they can fetch them directly!

So, while this is creating a difference, it's allowing the code in a GDExtension to effectively be more similar to a module's, by allowing it to access singletons at all during this level.

EDIT: Marking this as a draft currently, as this is currently just a proposal for discussion on the GDExtension team. Will take it out of draft if folks think this makes sense!

[dsnopek]

Closing this for now, as I think the first step is more discussion

[Bromeon]

Related: ThemeDB is not available during initialization time, as it's registered in register_scene_singletons() -- which is called after the MODULE_INITIALIZATION_LEVEL_EDITOR stage.