[slightlyoff/ServiceWorker] Allow caches to opt-in to granular cleanup (#863)

Something like:

```js
cache.settings({
  removalStrategy: "least-matched"
});
```

…where the developer can allow the browser to auto-delete items from the cache based on some kind of strategy.

Apologies for the really rough idea, just wanted to add something before I forgot :smile: 

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