Update adapter-vercel to use new build output API

[Rich-Harris]

A new version of the build output API (formerly 'filesystem API') is coming soon. Projects can opt into it with ENABLE_VC_BUILD=1. This PR adds support for it, by checking for that environment variable and using the old API if it's not being used.

The code is repetitive, which I think is fine — we'll rip out the v1 branch as soon as v3 is out of beta.

• add edge option
• add split option
• allow functions to handle /__data.json requests

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🦋 Changeset detected

Latest commit: f08755c

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

This PR includes changesets to release 2 packages

Name	Type
@sveltejs/adapter-vercel	Patch
@sveltejs/kit	Patch

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