Allow port 0 in `next dev` and `next start` (#40118)
This addresses a bug where invoking `next dev` or `next start` with `--port 0` would fall back to the default port of 3000 instead of binding to port 0 (which typically results in the operating system assigning a free port).
I couldn't find a straightforward way of adding a test for next-start. It looks like we could add a similar test as for dev, but would need to generate a built project to serve.
Manual test plan for `next start`:
```
$ ./packages/next/dist/bin/next start --port 0
ready - started server on 0.0.0.0:53508, url: http://localhost:53508
```
## Bug
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [x] 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)
Co-authored-by: Steven <steven@ceriously.com>
Co-authored-by: JJ Kasper <jj@jjsweb.site>
Co-authored-by: Wyatt Johnson <accounts+github@wyattjoh.ca>
