child_process: refactor stdioStringToArray function

reduce the function in both files to one.

PR-URL: https://github.com/nodejs/node/pull/27657


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>