[adapter-vercel] don't use top level await in Vercel Edge Functions

[Schniz]

This fixes a regression created in #6327, due to the usage of top level await, which isn't supported in Vercel Edge Functions right now.
Using a deferred promise and awaiting within the response it practically the same thing, but works.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 5955b60

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@sveltejs/adapter-vercel	Patch
@sveltejs/adapter-netlify	Patch
@sveltejs/adapter-auto	Patch

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[dummdidumm]

Thank you!