Accept Unix Sockets as PORT for remix-serve

[fabianeichinger]

I've to run a Remix App in an environment where HTTP requests are forwarded via a Unix Socket file from the reverse proxy. The path to the socket is provided in the PORT environment variable instead of a TCP port number.

Currently, remix-serve falls back to automatically assigning a TCP port when PORT can't be coerced to a number. It was fairly easy to fix up https://github.com/remix-run/remix/blob/main/packages/remix-serve/cli.ts to also support socket files (Node's http.server accepts either port or socket file) but I'd prefer not having to maintain more explicit dependencies.