[whatwg] Mechanism to find available events

[Biju]

https://bugzilla.mozilla.org/show_bug.cgi?id=264525

We need
HTMLNode.getSupportedEvents()  ==> returns a text array of event names
HTMLNode.isSupportedEvent(eventName)  ==> returns true/false

Many times in particular version of browser we dont know whether an
HTMLNode/window support particular event.
Or what are the alternate events available, so that we can refer some
document or do some test to find how it can be used.
So I wish we had above methods available.