process: move child process IPC setup condition into node.js

Instead of branching in main_thread_only.js, move the branch on
process.env.NODE_CHANNEL_FD in node.js so it's easier to tell when
this needs to happen. Also added comments about what side effect
this causes, and lazy load `assert`.

PR-URL: https://github.com/nodejs/node/pull/25130


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>