doc: add fs declarations to stream doc js examples

PR-URL: https://github.com/nodejs/node/pull/18804


Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>