feat: use web standard fetch export

[Rich-Harris]

Vercel now supports using web standard requests and responses in serverless functions, which means we no longer need to muck around with getRequest and setResponse

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@sveltejs/adapter-vercel	Minor

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[Rich-Harris]

Verified that both SSR and ISR requests succeed (sveltejs/svelte.dev#1481)

[elliott-with-the-longest-name-on-github]

Love it!