Manipulate non-send resources with Commands

[BD103]

Objective

• While it is possible to manipulate Resources using Commands, it is not possible to do the same for non-send resources (NonSend).
  • Please see Upstream bevy_command_non_send #12795 for full justification and additional discussion.
• Closes Upstream bevy_command_non_send #12795.
• Incompatible with Take non-Send resources out of World #9122. I'd prefer that get merged instead of this one, but... uhhh...

Solution

• Implements init_non_send_resource, insert_non_send_resource, and remove_non_send_resource methods for Commands.
  • Since Commands is accessible across threads, insert_non_send_resource's implementation takes a closure that creates the non-send resource on the main thread, instead of straight-up taking the value. This can be a bit confusing, so please check my documentation closely to make sure I explained it well!
• I additionally moved remove_resource in 13e0f17, which is why the diff looks weird. I recommend looking at 314391c for the actual new content.

---

Changelog

• Added methods for manipulating non-send resources in Commands.

Migration Guide

This PR is not actually a breaking change, but I feel a migration guide is useful. Comment if you think I should remove it. :)

It is now possible to insert and remove non-send resources using Commands. If you have any systems that take &mut World like this:

fn insert_non_send(world: &mut World) {
    world.insert_non_send_resource(MyNonSend::new());
}

You can replace it with the following, which may yield performance improvements:

fn insert_non_send(mut commands: Commands) {
    commands.insert_non_send_resource(|| {
        MyNonSend::new()
    });
}

This also applies to the methods init_non_send_resource and remove_non_send_resource. Note that Commands::insert_non_send_resource takes a closure, not MyNonSend directly. This closure's returned value will be inserted, after it is run on the main thread.

Be warned that this may change the order of inserting your resource! Commands works by delaying work until apply_deferred is called. If you depend on a resource being inserted / removed within a schedule, using Commands will break your code.

[NthTensor]

Tentatively marking this as controversial, as 9122 is supposed to be out of draft soon and seems to have broad support.

@SkiFire13 asks on discord:

12819 says it is incompatible with 9122 but is that really the case? Couldn't the commands in 12819 be implemented using the TLS after 9122 is merged?

[BD103]

Closing this until #9122 is merged, where we can then reconsider it.