child_process: support promisified `exec(File)`

Author: Benjamin Gruenbaum <inglor@gmail.com>
Author: Anna Henningsen <anna@addaleax.net>

PR-URL: https://github.com/nodejs/node/pull/12442


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: William Kapke <william.kapke@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>