Implemented Resource for ()

[pablo-lua]

Objective

• Closes Implement Resource for () #14640

Solution

• Implement Resource for (), like the first version of Draft: Implement Resource for () #14877
  Note that this PR doesn't implement ReflectResource for Unit type: There isn't much a reason to do so.
  This can only affect you if you need to use () more than a resource, using it as a Reflected Resource too.

Testing

• Tested locally, with a system querying a Option<Res<()>>, as expected, returned None.

---

Showcase

// Let's say I have a system or plugin that I want the user to specify the Type of the Resource
fn my_system<T: MyTrait + Resource = ()>(resource: Option<Res<T>>) {
      // This will do nothing if the user defines so.
      // if None, not found or the dummy () type
      if let Some(res) = resource {
           res.my_trait();
      }
}

Notes

Comments in #14877 suggests adding a MaybeResource type query.
Even so, its simpler and more ergonomic IMO to just implement () as a Resource, so you can use default types without having to create ZST to do this work for you. But have the drawback of you having to use Option to intermediate between a type that exists in the world, or the dummy () type.

World::resource::<()> will not work: () isn't really inserted in the World, instead you should use Option or get_resource() to help you here.

[pablo-lua]

Nevermind: Never noticed that this approach was banned basically because of SceneSpawner notices that there is a type not registered as ReflectResource, which is a problem with this approach. Making implementing Reflect for unit is possible, but isn't good. In all worlds
Two solutions possible are the MaybeResource type query, or implement Resource for unit type, but not inserting in all Worlds.
Its possible to relax the requirements by using another approach.