Better hooks for functional events

Most specifications that define functional events don't seem to fire them with the arguments Service Workers prescribes. In particular postDispatchSteps is often rather hand-wavy.

And given that some of the arguments are optional, but none are named (see Infra), it's rather unclear how to properly do this.

On top of that specifications might want to do something only after the events extension period is over, and perhaps do different things depending on whether the promise resolves or rejects. Doing this cleanly is somewhat murky at the moment. In Push API for instance we simply wait for some state to flip while in parallel: https://w3c.github.io/push-api/#dfn-fire-a-push-event

This should be organized a bit better.

Start by reading the issue's references to Infra and the Push API's “fire a push event” definition, then compare how functional events currently describe postDispatchSteps and optional arguments. Determine a consistent hook organization that covers the events extension period and promise resolution or rejection. Done means the specification guidance is concrete and consistently applicable to the affected event definitions.