Release Engine after unregistering GDExtensions

[raulsntos]

The Engine is used to retrieve singletons, and GDExtensions may try to retrieve a singleton (e.g.: OS) in their deinitialization.

MRP: https://github.com/raulsntos/GDExtensionSingletonCrash

The MRP shows the following errors and then crashes:

ERROR: Failed to retrieve non-existent singleton 'OS'.
   at: get_singleton_object (core/config/engine.cpp:294)
ERROR: Parameter "singleton_obj" is null.
   at: get_singleton (godot-cpp/gen/src/classes/os.cpp:47)

[dsnopek]

This makes sense to me.

Engine is created before initialize_modules(MODULE_INITIALIZATION_LEVEL_CORE), so it would make sense that it's deleted after uninitialize_modules(MODULE_INITIALIZATION_LEVEL_CORE), since we want to clean things up in basically the reverse order that we created them.

[akien-mga]

Thanks!