Editorial: queue a task to reject promise in InstallEvent.addRoutes

[monica-ch]

This is a small follow-up related to #1740.

The rejection path inside the [[service worker queue]] block of InstallEvent.addRoutes() was rejecting |promise| directly from inside the [[service worker queue]] (a parallel queue)". Rejecting a JS promise from a parallel queue crosses the event-loop/parallel boundary and is the same pattern that #1740 flags across the spec.

This PR:

• Wraps the rejection in a Queue a task on the service worker's event loop, using the DOM manipulation task source, matching the existing resolve path a few lines below.
• Adds an Abort these steps. after the rejection queue so we don't fall through to Set |serviceWorker|'s list of router rules and the follow-up resolve on the same promise.
• Hoists Let |serviceWorkerEventLoop| be the current global object's event loop. above the [=queue/Enqueue=] block so both the reject and resolve tasks can reference it (and so the event loop is captured on the event-loop side, not inside the parallel queue).

Editorial only, no normative behavior change beyond making the failure path stop after rejecting (which it should already do — the previous text would set the rules and resolve after rejecting, which was a bug in the failure path).

This is a follow-up spun out of #1755, InstallEvent.addRoutes had the same reject-in-parallel bug, but sits outside the sections covered by any of those.

Refs: #1740

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[monica-ch]

@yoshisatoyanagisawa Can you also review this when you get a chance!

[yoshisatoyanagisawa]

good catch!