`AppExit` should be an observer event rather than a `BufferedEvent`

[ItsDoot]

What problem does this solve or what need does it fill?

Multiple AppExit events in a buffer doesn't make sense, and would be better modeled as an observer event. This would be more performant for plugins to react to app exits, rather than having to schedule a system that runs every frame.

What solution would you like?

Swap AppExit to an observer event.

What alternative(s) have you considered?

Weird status quo.