feat: enables 'edge' as a possible runtime for API routes

[feugy]

📖 What's in there?

The edge runtime is going stable for API routes.
This PR brings a new allowed key inside pages/api/ folder:

const config = { runtime: 'edge' }

Regular pages can still be rendered using const runtime = 'experimental-edge', but they can not use edge(not all the cases are working yet).
This PR bring a new build error when using const runtime = 'edge'.

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• e2e tests added
• Documentation added (from PR 43814)
• Telemetry added. In case of a feature if it's used or not.
• Errors have a helpful link attached, see contributing.md

🧪 How to test?

Several tests cases added:

• in dev mode, errors and warning: NEXT_TEST_MODE=dev pnpm testheadless --testPathPattern edge-configurable-runtime
• in build mode, build error for pages on the edge: NEXT_TEST_MODE=start pnpm testheadless --testPathPattern edge-configurable-runtime

You can of course test it live inside the example/out folder:

// examples/out/stable-edge/pages/index.jsx
export default function Page() {
  return <p>hello world</p>
}
export const runtime = 'experimental-edge'; // try changing to `edge` to see the console error

// examples/out/stable-edge/pages/api/index.jsx
export default function handler() {
  return new Response('ok')
}
export const config = { runtime: 'edge' }; // try changing to `experimental-edge` to see the console warning

Then pnpm next build examples/out/stable-edge and pnpm next dev examples/out/stable-edge

[feugy]

Was ported and merged.