Expose Event, EventTarget to AudioWorklet

[hoch]

cc: @domenic @annevk

Related issue from Web Audio API

The proposed change is:

[Constructor(DOMString type, optional EventInit eventInitDict),
 Exposed=(Window,Worker,AudioWorklet)]
interface Event { ... }

[Constructor,
 Exposed=(Window,Worker,AudioWorklet)]
interface EventTarget { ... }

If this looks okay, I'll submit a PR shortly.

[annevk]

Looks good, thanks!