feat(EventDispatcher): expose protected listeners

[HoloTheDrunk]

Resolves #1374

Exposes the _listeners storage as a protected field so extending classes can access it.
The testsuite only worked once then never again, skipping with a fetch error on attw that I couldn't find anywhere online. I'm assuming this is due to the proxy at my work, but either way I can't test this myself at the moment. Seems to work though and it shouldn't be a breaking change.

[HoloTheDrunk]

Oof, seems like this is a much more significant change than I'd expected since nothing around listeners was strictly typed...
Might even be a breaking change considering that there was absolutely no type checking on event types or anything before.
I'll try to work through the errors one by one but help would be appreciated. This large node of missing typing being ignored for so long has led to a rather inconveniently large number of dependent code doing JS in TS.