[w3c/ServiceWorker] Add a script version property to the ServiceWorker object (#1387)

When a page with a controlling service worker calls `navigator.serviceWorker.register()`, there's no easy way for that page to know whether or not the script being registered will trigger an update (assuming the developer is [following best practices](https://developers.google.com/web/fundamentals/primers/service-workers/lifecycle#avoid-url-change) and not changing the script URL).

This is problematic if the page wants to communicate with the service worker via `postMessage` because whether or not the service worker will be able to respond to those messages depends on what version of the service worker is running. But from the page's perspective, there's no easy way to determine that.

**Proposal:**

A relatively simple solution to this would be to add an optional `scriptVersion` property to the options object you can pass to `register(scriptURL, options)`:

```js
navigator.serviceWorker.register('/sw.js', {
  scriptVersion: '1.0.0',
});
```

This `scriptVersion` would then be exposed on the `ServiceWorker` object, so you could reason about whether it's safe to communicate with the service worker from the page.

```js
if (navigator.serviceWorker.controller &&
    navigator.serviceWorker.controller.scriptVersion === '1.0.0') {
  // Do something now that you know it's safe.
}
```

You could also use the version info to determine whether or not an update is "significant" enough to display a notification to the user (using whatever versioning convention you use):

```js
const controller = navigator.serviceWorker.controller;
const registration = await navigator.serviceWorker.getRegistration();
const getMajorVersion = (v) => v.split('.')[0];

registration.addEventListener('updatefound', () => {
  if (getMajorVersion(registration.installing) > getMajorVersion(controller) {
    // Do something because now we know this is a "major" update.
  }
});
```

And this would be completely optional. If the user doesn't set a version, then it's simply not present on the `ServiceWorker` instance (or it's present but null).

If the user set a version that's already been registered, it would be an error. And if the user called register and passed a new `scriptVersion` but the script didn't update, it would also be an error (or perhaps an no-op).

An alternative to a user-submitted version would be a version automatically generated by the UA, but I'd argue that's less useful since then you'd have to store that version in localStorage or IDB in order to use it across page loads. If you wanted to have something unique to the instance, you could have a separate `id` value that's auto-generated or an initial registration `timeStamp` property that would also be unique.




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