refactor: consolidate all events in Events.js (#3772)

This will allow us to break all dependency cycles that were forcing
us to put many things in a single file (e.g. ExecutionContext and
ElementHandle).

utils/doclint/check_public_api/test/diff-events/foo.js

@@ -1,7 +1,3 @@
 class Foo {
 }
 
-Foo.Events = {
-  Start: 'start',
-  Finish: 'finish',
-};