Schedule resource mutation

[lee-orr]

Objective

Resolves #13185

Solution

Move the following methods from sub_app to the Schedules resource, and use them in the sub app:

• add_systems
• configure_sets
• ignore_ambiguity

Add an entry(&mut self, label: impl ScheduleLabel) -> &mut Schedule method to the Schedules resource, which returns a mutable reference to the schedule associated with the label, and creates one if it doesn't already exist. (build on top of the entry(..).or_insert_with(...) pattern in HashMap.

Testing

• Did you test these changes? If so, how? Added 4 unit tests to the schedule.rs - one that validates adding a system to an existing schedule, one that validates adding a system to a new one, one that validates configuring sets on an existing schedule, and one that validates configuring sets on a new schedule.
• I didn't add tests for entry since the previous 4 tests use functions that rely on it.
• I didn't test ignore_ambiguity since I didn't see examples of it's use, and am not familiar enough with it to know how to set up a good test for it. However, it relies on the entry method as well, so it should work just like the other 2 methods.

[alice-i-cecile]

I agree with moving these up a level. Nice and simple and a more reasonable API.

[Trashtalk217]

This is a good change. There was very little reason for these methods to be exlusive to bevy_app.

[MiniaczQ]

LGTM