Make WorldId a SystemParam

[alice-i-cecile]

          While I don't think this is a bad idea, the justification of using `Local<WorldId>` has me set off a bit (akin to how `Query<Option<With<T>>>` does). Could we not make `WorldId` a `SystemParam`?

Originally posted by @james7132 in #7726 (review)

This should be able to draw directly from the implementation in the linked PR: it should return the WorldId of the world that the system is running on.

[LiamGallagher737]

I have tested this, and it seems to work in practice, but I have no idea if it's safe or if it's the correct way to do this.

/// SAFETY: No idea
unsafe impl ReadOnlySystemParam for WorldId {}

/// SAFETY: No idea
unsafe impl SystemParam for WorldId {
    type State = WorldId;

    type Item<'world, 'state> = WorldId;

    fn init_state(world: &mut super::World, _: &mut crate::system::SystemMeta) -> Self::State {
        world.id
    }

    unsafe fn get_param<'world, 'state>(
        _: &'state mut Self::State,
        _: &crate::system::SystemMeta,
        world: &'world super::World,
        _: u32,
    ) -> Self::Item<'world, 'state> {
        world.id
    }
}

[james7132]

That looks generally correct to me beyond the safety comments. Something along the lines of read-only access to shared World metadata for ReadOnlySystemParam, and A World's ID is immutable and fetching it from the World does not borrow anything for SystemParam should work, bar some wordsmithing.

Another alternative is to just read it from the System itself, since systems are bound to the World they're initialized on.