[w3c/ServiceWorker] Editorial: queue a task to reject promise in InstallEvent.addRoutes (PR #1839)

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

The rejection path inside the `[[service worker queue]]` block of `InstallEvent.addRoutes()` was calling `Reject |promise|` directly from 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).

Refs: #1740


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You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/ServiceWorker/pull/1839

-- Commit Summary --

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

-- File Changes --

    M index.bs (7)

-- Patch Links --

https://github.com/w3c/ServiceWorker/pull/1839.patch
https://github.com/w3c/ServiceWorker/pull/1839.diff

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