Re: [w3c/manifest] Add offline_url option (#774)

Looks like the Declarative routing (w3c/ServiceWorker#1373) would be able to support this use case without adding the overhead of a fetch handler.

So the example code I wrote above for the hypothetical `addOfflineHandler`:

```js
self.addEventListener('install', () => {
  self.addOfflineHandler("/offlinepage.html",
     {resources: ["/assets/heading.png", "/assets/style.css"]});
});
```

Could be expressed with declarative routes (making use of the "Extensibility" extensions) as:

```js
self.addEventListener('install', event => {
  router.add(
    new RouterOr(new RouterIfURL('/assets/heading.png'), new RouterIfURL('/assets/style.css')),
    new RouterSourceCache());
  router.add(
    new RouterIfURLStarts('/'),
    [new RouterSourceNetwork(), new RouterSourceCache('/offlinepage.html')]);

  event.waitUntil(
    (async () => {
      const cache = await caches.open(CACHE_NAME);
      await cache.addAll([
        '/assets/heading.png',
        '/assets/style.css',
        '/offlinepage.html',
      ]);
    })()
  );
});
```

Obviously that's a lot more verbose, but it could be wrapped in a library pretty simply.

I don't like having to use the extremely verbose `RouterOr` just to do the same logic but with different exact URLs. Easily fixed with a `RouterIfURLOneOf(['...', '...'])` (or just allow `RouterIfURL` to take a list).

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