Make bevy_app and reflect opt-out for bevy_hierarchy.

[mintlu8]

Objective

Bevy_hierarchy is very useful for ECS only usages of bevy_ecs, but it currently pulls in bevy_reflect, bevy_app and bevy_core with no way to opt out.

Solution

This PR provides features bevy_app and reflect that are enabled by default. If disabled, they should remove these dependencies from bevy_hierarchy.

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Changelog

Added features bevy_app and reflect to bevy_hierarchy.

[alice-i-cecile]

Well done. This is something I'd like to see more of in our crates :)

Getting the config right for the optional dependencies is tricky, but it looks to me like you've done it. Criteria for me are:

1. Nice granular features.
2. Same default behavior.
3. No default features works well with raw bevy_ecs.

[alice-i-cecile]

It's complaining about the formatting of your toml files: you'll have to clean that up before we can merge :)

Ah, it's actually another problem.