fix: apply default export interop to `next/config` (#40224)
The `next/config` code generated is incompatible with the `type: "module"` setting in `package.json`. This PR makes sure that we append the same interop code to the output `shared/lib/runtime-config` file as other re-exported modules:
https://github.com/vercel/next.js/blob/93830bf04f1f40e1996f21a3534b6010d9ef3c0c/packages/next/taskfile-swc.js#L124-L128
Fixes #40159
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] 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`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
