child_process: create proper public API for `channel`

Instead of exposing the C++ bindings object as `subprocess.channel`
or `process.channel`, provide the “control” object that was
previously used internally as the public-facing variant of it.

This should be better than returning the raw pipe object, and
matches the original intention (when the `channel` property was
first added) of providing a proper way to `.ref()` or `.unref()`
the channel.

PR-URL: https://github.com/nodejs/node/pull/30165
Refs: https://github.com/nodejs/node/pull/9322
Refs: https://github.com/nodejs/node/issues/9313


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>