[FEAT] Enter/Exit queries

[JaimeGensler]

Describe the problem this feature solves

Queries for when an entity beings matching some conditions, or no longer matches some conditions

Describe the solution you'd like to see

Filters (Enter<T>/ Exit<T>) are one solid option and is how Bevy handles this. I think there's a solid argument to be made that these are conceptually a bit different from normal queries (it's more like an event listener, really), so a new type of query-like system parameter also could make sense.