[whatwg/dom] Property to distinguish whether listener is passive or no/event is preventable (#587)

Linking issue from interventions WICG repo to keep continuity: WICG/interventions#63.

Summarizing:

There is no way to distinguish (from **inside of the listener**) whether event can be cancelable or no (or simpler - if preventDefault can be fired) - unfortunately `cancelable` property, which is available in the event object doesn't give any info about that (at least in scenario of passive listener).

Proposal would be to change a bit conditions under which cancelable is set to false (to cover passive listener scenario) or to add to the event object a new property (e.g. 'preventable') which will always go along with the real possiblity to stop the event.

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