Add option to add_class at init time level

[Ughuuu]

I want to register a PhysicsServer at InitLevel::Servers, and in order to do so, I need to do:

pub fn register() {
    let mut manager = PhysicsServer2DManager::singleton();
    let factory = RapierPhysicsServerFactory2D::new_alloc();
    manager.register_server("Rapier2D".into(), factory.callable("create_server"));
}

For this, I need to create a new node, eg. RapierPhysicsServerFactory2D that will create the server(as such is the API of the PhysicsServer2DManager). In order to do so at InitLevel::Servers level, I need to be able to register nodes before it.

[lilizoey]

This is actually a bug too, we set a lot of the classes to have init level = Scene. even things that shouldn't like the physics server and physics server extensions. editor classes have Editor as they should though