* events: Refactor; move Event into core, so core can emit events
Requires some slight trickery to invert dependencies. We can't have the caddy package import the caddyevents package, because caddyevents imports caddy. Interface to the rescue!
Also add two new events, experimentally: started, and stopping. At the request of a sponsor.
Also rename "Filesystems" to "FileSystems" to match Go convention (unrelated to events, was just bugging me when I noticed it).
* Coupla bug fixes
* lol whoops
