Turbopack: Document automatic Babel config support (#84002)
~I'm looking for review/feedback, but **DO NOT MERGE UNTIL NEXT 16.0 IS RELEASED.** This is not supported on 15.5, and merging these docs now would be confusing to users. If you want to try it out, use a recent canary release.~ I've been informed that docs are not currently tracking canary, so this is safe to merge.
https://github.com/vercel/next.js/pull/82676 adds support for automatically enabling Babel if a babel config is found. Previously we'd exit the process with a hard error explaining that Babel is not supported.
Technically you could previously manually configure `babel-loader`, but we had heuristics to detect and warn against that, and you'd need to use a different path for the config file to work around our error message, so it didn't really work properly. (This seems like it was a mistake/oversight)
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