Event: Restore the `constructor` property on jQuery.Event prototype

The original definition of the jQuery.Event prototype was paving over the
`constructor` property which was causing jQuery.isPlainObject to
improperly report that an instance of jQuery.Event was a plain object.

Fixes #15090
Closes gh-1580

(cherry picked from commit b807aedb7fee321fb3aa5d156a5a256ab0634e2d)