Using System for Trigger

[hyranno]

Hello.
Bevy has its own mechanics to handle functions with SystemParam and corresponding SystemState.
This PR aim to

• use it in TransitionImpl rather than implement
• add interoperability for System and Trigger

The interoperability implemented as wrappers: TriggerSystemFunction and SystemFunctionTrigger.
I'm not so sure this is good implement.
I couldn't

• implement IntoSystem on T: Trigger directly
• implement without breaking change

Breaking Change

Trigger::trigger(&self, ...) -> Trigger::trigger(&mut self, ...)

[Seldom-SE]

Thanks for the pr, my life is busy right now, but I'll take a look when I'm able

[Seldom-SE]

I have some plans for an update (after 0.12 releases) that conceptually conflict with this, but I'll leave this open until I confirm that such plans are actually a good idea

[Seldom-SE]

Closing in favor of dec8a58