feat: add opt-out support for bevy_eventlistener

[musjj]

This adds support for bevy_eventlistener, which allows users to attach callbacks to entities and have it react to events targeting that entity.

This doesn't affect users who don't use it and they will still be able to listen to the events normally using EventReader<SpanTweenerEnded>. So I made the feature opt-out, but I'm open to making it an opt-in too if there are good reasons to.

[Multirious]

I usually preferred 3rd party crate to be opt-in but eventlistener does goes really well with the design of this crate. Opt-out should be good.

[Multirious]

Thanks!