Re: [w3c/ServiceWorker] Declarative routing (#1373)

Really liking the concept of avoiding the overhead of starting the ServiceWorker where possible, but I feel like not adding an additional Source type that utilises a callback is a missed opportunity. It would effectively be the same behaviour as the fetch event fallback, but specific to the given conditions. For all other source types we would still avoid starting up the service worker.

Also I think syntax wise I feel it reads better to have static methods on a RouterCondition/RouterSource class that instantiate the specific type. Although I'm not sure how in keeping that is with other web APIs.

```javascript
const { router } = e;

router.get(
  RouterCondition.startsWith("/avatars/"),
  [ RouterSource.cache(), RouterSource.network() ]
);

router.get(
  RouterCondition.fileExtension(".mp4"),
  RouterSource.network()
);

router.get(
  RouterCondition.URL("/"),
  RouterSource.cache("/shell.html")
);

router.add(
  RouterCondition.any(),
  RouterSource.custom(fetchEvent => {
    
  })
);
```

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