feat: implement skew protection

[Rich-Harris]

closes #10947

This implements Vercel Skew Protection. Tested manually

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Latest commit: d9bb954

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@sveltejs/adapter-vercel	Minor

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[leerob]

Woohoo! Thank you for implementing this 🥳

[dummdidumm]

previously you deleted the cookie for the version.json route - is that not necessary after all? Not necessary because it's not a document request?

[Rich-Harris]

correct — we only care about non-document requests for this file. AFAICT there's no way to set multiple cookies (you can't do set-cookie: [...], and if you add a header in multiple continue routes the last one wins), so we had to get rid of it

[Rich-Harris]

wait, no, hang on... this is a problem — the version.json will be requested with the old deployment ID. gah

[Rich-Harris]

(this seemed to work in my testing, because the cookie was still in my browser from the earlier iteration 🤦 )

[Rich-Harris]

I've 'fixed' this by setting the second set-cookie header on the response for the entry point, since we know it will happen before the client hydrates. Since adapters don't have access to the build manifest, this involves some hackery, but hopefully we can remove it in future once the Build Output API supports setting multiple set-cookie headers in a single response.