fs,net: standardize `pending` stream property

Use the same property name as http2 does to indicate that
the stream is in the state before the `ready` event is emitted.

PR-URL: https://github.com/nodejs/node/pull/24067


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: James M Snell <jasnell@gmail.com>