fs: use encoding in readFile

Use the encoding parameter passed to fsPromises.readFile if it is
passed. Currently the encoding parameter is ignored in fsPromises.

PR-URL: https://github.com/nodejs/node/pull/19296
Fixes: https://github.com/nodejs/node/issues/19286


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>