Statically designate event targets

[alice-i-cecile]

Originally posted by @cart in #16029

I don't think Option<Entity> is the right approach here. Events (or whatever we choose to call them) should be designed with a specific target type in mind (no target, entity target, component target, etc).

Right after we landed observers I implemented "static event targets", which removes the need for Option / PLACEHOLDER , makes it statically impossible for users to try to access a non-existent entity for a "global event", and general prevents expressing the "wrong" thing.

https://github.com/cart/bevy/tree/static-event-targets

[alice-i-cecile]

Merging #14843 and #14272 into this issue.