Add labels for `scene_spawner_system` and `scene_spawner`

[Shatur]

What problem does this solve or what need does it fill?

When users want to order something before scene spawning, they need to use the system name directly:

        .add_systems(
            SpawnScene,
            wait_for_request
                .before(scene::scene_spawner_system)
                .run_if(in_state(PreviewState::Inactive)),
        )

What solution would you like?

It would be nice to have an enum label for scene_spawner_system and scene_spawner (yes, there are two systems with similar names), similar to TransformSystems and others.
I'd also suggest to give them proper names.