[slightlyoff/ServiceWorker] Add a way to detect reloading only client (#872)

Client lifetimes overlap when reloading a page, so at the "navigate" request after pressing reload, there will appear to be two clients (or just the old one because clientId is null? not clear on this). This makes it hard to know if the page is being reloaded with no other clients open, which is an opportunity to do an update if some new data has been downloaded. I think there should be a way to easily detect this case.

I don't think the existing `isReload` property tells you much, since it doesn't appear to tell you anything about other clients. Also I am not that familiar with SW so forgive me if this is already possible some way!

For example maybe the "navigate" fetch event could have `previousClientId`. There would probably need to be a non-null `clientId` in the navigate event as well for this to be useful. This would mean that:

- if this is a "navigate" fetch,
- `isReload` is true,
- `clients.matchAll()` consists only of `previousClientId` and the new `clientId`
- I know that my SW will serve a document that replaces the previous client and doesn't start a download or otherwise leave behind the old client

...then it is safe to update if new data is waiting.

Alternatively maybe there could be a separate `isOnlyClientReload` flag to detect the same?

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