Add a scope API for world schedules

[JoJoJet]

Objective

If you want to execute a schedule on the world using arbitrarily complex behavior, you currently need to use "hokey-pokey strats": remove the schedule from the world, do your thing, and add it back to the world. Not only is this cumbersome, it's potentially error-prone as one might forget to re-insert the schedule.

Solution

Add the World::{try}schedule_scope{ref} family of functions, which is a convenient abstraction over hokey pokey strats. This method essentially works the same way as World::resource_scope.

Example

// Run the schedule five times.
world.schedule_scope(MySchedule, |world, schedule| {
    for _ in 0..5 {
        schedule.run(world);
    }
});

---

Changelog

Added the World::schedule_scope family of methods, which provide a way to get mutable access to a world and one of its schedules at the same time.

[alice-i-cecile]

Very nice: I was hoping we'd get these.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[hymm]

I'm not a big fan of the proliferation of the _ref functions, given the footgun with double boxing the label. Code quality looks good otherwise.

[JoJoJet]

I'm not a big fan of the proliferation of the _ref functions, given the footgun with double boxing the label. Code quality looks good otherwise.

I agree. I think we should merge the ref variants with the owned versions of these functions, but I'm including them for now for consistency with how {try_}run_schedule already works.