Allow `next.config.mts` for Node.js native TS resolver (#83556)
Following up on https://github.com/vercel/next.js/pull/83240
When using ESM syntax in `next.config.ts` on a CJS project, Node.js
first assumes it as CJS, then parses it into ESM again, giving a
warning:
```
(node:27529) [MODULE_TYPELESS_PACKAGE_JSON] Warning: Module type of file://.../next.config.ts is not specified and it doesn't parse as CommonJS.
Reparsing as ES module because module syntax was detected. This incurs a performance overhead.
To eliminate this warning, add "type": "module" to .../package.json.
```
Therefore, give users the option to use `next.config.mts` which Node.js
expects ESM syntax.
The tests are a copy of `next-config-ts-native-ts` and only renamed the
config to mts.
---------
Co-authored-by: Sebastian "Sebbie" Silbermann <sebastian.silbermann@vercel.com>
