Move some structs that impl Command to methods on World and EntityWorldMut

[JaySpruce]

Objective

Commands were previously limited to structs that implemented Command. Now there are blanket implementations for closures, which (in my opinion) are generally preferable.

Internal commands within commands/mod.rs have been switched from structs to closures, but there are a number of internal commands in other areas of the engine that still use structs. I'd like to tidy these up by moving their implementations to methods on World/EntityWorldMut and changing Commands to use those methods through closures.

This PR handles the following:

• TriggerEvent and EmitDynamicTrigger double as commands and helper structs, and can just be moved to World methods.
• Four structs that enabled insertion/removal of components via reflection. This functionality shouldn't be exclusive to commands, and can be added to EntityWorldMut.
• Five structs that mostly just wrapped World methods, and can be replaced with closures that do the same thing.

Solution

• Observer Triggers (observer/trigger_event.rs and observer/mod.rs)
  • Moved the internals of TriggerEvent to the World methods that used it.
  • Replaced EmitDynamicTrigger with two World methods:
    • trigger_targets_dynamic
    • trigger_targets_dynamic_ref
  • TriggerTargets was now the only thing in observer/trigger_event.rs, so it's been moved to observer/mod.rs and trigger_event.rs was deleted.
• Reflection Insert/Remove (reflect/entity_commands.rs)
  • Replaced the following Command impls with equivalent methods on EntityWorldMut:
    • InsertReflect -> insert_reflect
    • InsertReflectWithRegistry -> insert_reflect_with_registry
    • RemoveReflect -> remove_reflect
    • RemoveReflectWithRegistry -> remove_reflect_with_registry
• System Registration (system/system_registry.rs)
  • The following Command impls just wrapped a World method and have been replaced with closures:
    • RunSystemWith
    • UnregisterSystem
    • RunSystemCachedWith
    • UnregisterSystemCached
  • RegisterSystem called a helper function that basically worked as a constructor for RegisteredSystem and made sure it came with a marker component. That helper function has been replaced with RegisteredSystem::new and a #[require].

Possible Addition

The extension trait that adds the reflection commands, ReflectCommandExt, isn't strictly necessary; we could just impl EntityCommands. We could even move them to the same files as the main impls and put it behind a #[cfg].

The PR that added it had a similar conversation and decided to stick with the trait, but we could revisit it here if so desired.

[LikeLakers2]

When I read the title and PR description, I was initially concerned that you were removing functions from Commands.

However, I read through the code, and I'm glad to see that isn't the case - you were simply changing their code from queuing up a impl Command struct, to queuing up a closure.

I'm not a bevy team member, but I approve this nonetheless.

[alice-i-cecile]

@LikeLakers2 thanks for the review here: that's exactly the pattern of community review that Bevy is built around :)