refactor(dev): use Next.js externalDir to fix dev app

[balazsorban44]

This fixes the developer application after the TS refactor #2552

We rely on the experimental externalDir feature of Next.js found here: vercel/next.js#22867

This basically means we can import next-auth source code - without needing to compile it first - into the dev server even if it lives outside of Next.js's root folder, and HMR will "just work".

The only exception is the CSS which we use in Server-Side rendered pages that won't hot-reload anyway. They generate HTML through API routes, so no getServerSideProps is involved. This means editing the CSS won't trigger a manual page reload, but that has been the case before this PR as well.

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[ubbe-xyz]

Very neat 💯 💪🏽

[ndom91]

Yeah awesome solution!