feat: update service worker when new version is detected

[dumbmoron]

this patch adds a function that tries to update the service worker (if it exists), and calls it when sveltekit detects that a new version has been deployed.
i'm not sure if this is the right place to put this -- i considered putting it in the body of check(), but it feels wrong to me. if you think it's a good idea, let me know and i'll update it.

closes #3667

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[eltigerchino]

I don't have much knowledge about service workers but it looks good to me. cc: @dummdidumm what do you think?

[dummdidumm]

Thank you!