feat: expose waitUntil also for serverless runtime & add docs

[Fruup]

Fixes #12324.

This PR removes Vercel's context (as it is deprecated) and exposes the waitUntil function to the edge and serverless runtime.

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🦋 Changeset detected

Latest commit: bac9956

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-vercel	Minor

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-14725-svelte.vercel.app/