[w3c/ServiceWorker] Ground event listener existence check in new primitive (#1342)

https://github.com/whatwg/dom/pull/660 adds a new primitive to the DOM Standard to be used by other standards that need to observe event listeners for legacy reasons: "have a listener for".

I'd appreciate if you could check that this is sufficient for your needs.

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https://github.com/w3c/ServiceWorker/issues/1342