__are_oneditor_fields_initalized needs explicit usage of Singleton trait

[TitanNano]

In __are_oneditor_fields_initalized that is generated by the GodotClass derive macro, we use ::godot::class::Engine::singleton(). Since the introduction of the Singleton trait, we have to now explicitly mention the trait. Otherwise, users might run into compiler errors if they don't have the trait in scope:

no function or associated item named singleton found for struct godot::classes::Engine in the current scope
items from traits can only be used if the trait is in scope.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1400