The `WorkboxLifecycleEvent.isUpdate` is false on first immediate service worker update

**Library Affected**:
workbox-window

**Browser & Platform**:
At least Chrome 131.0.6778.86, possibly all browsers

**Issue or Feature Request Description**:

The `WorkboxLifecycleEvent.isUpdate` will remain `false` in following scenario:

1. New service worker installation
2. Alter source code to trigger update in next step
3. Manually trigger Service worker update (without navigation/ page reload), for ie. by one of;
- DevTools > Application > Service workers > Update
- `(await navigator.serviceWorker.getRegistration()).update()`

This matches the behavior described in source:

https://github.com/GoogleChrome/workbox/blob/c77dceb54d4af1749db95316710d6430e82b0c48/packages/workbox-window/src/Workbox.ts#L698-L699

However it's name leads to false assumption that it's value is `true` on *any update* (even that there was no controlling service worker at the time of registration).

Some examples:

- chrome for developers > Workbox-window docs > [When an updated version of the service worker is found](https://developer.chrome.com/docs/workbox/modules/workbox-window#when_an_updated_version_of_the_service_worker_is_found):

> When a new service worker starts installing but an existing version is currently controlling the page, the isUpdate property of all the following events will be true.

- web.dev > Broadcast updates to pages with service workers > [Listen to service worker lifecycle events](https://web.dev/articles/broadcast-updates-guide#listen-to-service-worker-lifecycle-events)

```ts
wb.addEventListener('installed', (event) => {
if (event.isUpdate) {
// Show "Update App" banner
}
});
```

- demos > [workbox-window](https://github.com/GoogleChrome/workbox/blob/v7.3.0/demos/src/workbox-window/index.html)

https://github.com/GoogleChrome/workbox/blob/c77dceb54d4af1749db95316710d6430e82b0c48/demos/src/workbox-window/index.html#L53-L57

**Suggested solution**

I think the fix is to set `this._isUpdate = true` after first installation:

```diff
diff --git a/packages/workbox-window/src/Workbox.ts
private readonly _onControllerChange = (originalEvent: Event) => {
const sw = this._sw;
const isExternal = sw !== navigator.serviceWorker.controller;

+ this._isUpdate = true
+ this._isUpdate = navigator.serviceWorker.controller !== null // Alternative implementation
```

and adjust jsdoc comments for this property.

I'm just not entirely sure if this is the right place?

Related PR: https://github.com/GoogleChrome/workbox/pull/1905