chore: lint vercel files

[teemingc]

The Vercel server handler files are in a files directory. This directory is where bundled output goes into for all the other adapter packages except for the Vercel adapter. As a result, it was accidentally included in the eslint ignore list.

This PR changes that so it's no longer ignored (alternatively, we should just rename it as src/?)

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 51458c4:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/51458c44c2d571840a1b163eee64e91887d5953b

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16487

[changeset-bot]

⚠️ No Changeset found

Latest commit: 51458c4

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16487-svelte.vercel.app/

[Rich-Harris]

it's the equivalent of the generated files in e.g. adapter-netlify or -cloudflare, the difference being that in those cases files is generated from src. So I think it's probably best to keep it like this