Check for browser-sync port availability

[derweili]

If the browser-sync port is already in use when running npm start the script does not throw an error or a warning.

I think it would be better when the script would either abort execution with an error message or switch to a different port (automatically or after user consent).

[swashata]

Hello,

This seems like a good idea and I would be open to a PR 👍. Thank you very much!