[ServiceWorker] Add cookie accessor/setter methods? (#707)

In keeping with our approach of only adding APIs as-needed, we've had a request come in from @cramforce for access to cookies from the SW context.

We'll need something asynchronous; e.g.:

```js
self.getCookies().then(...);
self.setCookies().then(...);
```

I'm afraid it won't lock, so might get out of sync with documents more easily than script access does today, but we should perhaps try it out and see how it goes.

While we're at it: do we want more structured cookie types for both these records and the cookies entries in `Response` objects?

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/707