Implement Turbopack build marker via manifest prop

[mischnic]

Implements detection for next build --turbopack to automatically set TURBOPACK=1.

When running next build --turbopack Next.js will set a property in .next/required-server-files.json which can be used to detect if the build used Turbopack.

At runtime Next.js needs TURBOPACK=1 in order to load the correct JS runtime that is compatible with Turbopack.

The environment variable will be required until Turbopack is the default build output of Next.js.

Requires vercel/next.js#77851
Closes #13211 , which used a separate marker files

[changeset-bot]

⚠️ No Changeset found

Latest commit: 8c64357

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.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR