Update next.config.js (#33091)
## Feature
We are in a Typescript context, but the `next.config.js` cannot be parsed as is, since it fails on `File is a CommonJS module; it may be converted to an ES module.ts(80001)` so it does not parse the rest of the file :

Here we should have an error because the type is not the expected one.
With a little trick, it's possible to enable the check by :
- adding `// @ts-check` at first line
- moving the export at the end of the file
And then it works like a charm :

Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>