Move utilities from examples to bevy_state and add concept of state-scoped entities

[MiniaczQ]

Objective

Move StateScoped and log_transitions to bevy_state, since they're useful for end users.

Addresses #12852, although not in the way the issue had in mind.

Solution

• Added bevy_hierarchy to default features of bevy_state.
• Move log_transitions to transitions module.
• Move StateScoped to state_scoped module, gated behind bevy_hierarchy feature.
• Refreshed implementation.
• Added enable_state_coped_entities<S: States>() to add required machinery to App for clearing state-scoped entities.

Changelog

• Added log_transitions for displaying state transitions.
• Added StateScoped for binding entity lifetime to state and app enable_state_coped_entities to register cleaning behavior.

[orbachl-mcmaster]

I agree with Alice's comments, and otherwise am happy with this!

[alice-i-cecile]

I've added S-Needs-Release-Notes to this PR. IMO this deserves its own (adjacent) section in the release notes. The functionality is distinct and can be used without substates / computed states, even though it does play nice.

[mockersf]

scoped entities should work without bevy_hierarchy, by despawning entities not recursively, and the doc for enable_state_scoped_entities should mention that behaviour

as it is currently, it should at least mentions that it despawns recursively

[alice-i-cecile]

Thank you to everyone involved with the authoring or reviewing of this PR! This work is relatively important and needs release notes! Head over to bevyengine/bevy-website#1347 if you'd like to help out.