src: add internalBindings for binding isolation

This commit adds a method to internal/process that allows access to
bindings that are not intended to be used by user code. It has a
separate cache object and modlist in order to avoid collisions.

You can use NODE_MODULE_CONTEXT_AWARE_INTERNAL to register a C++
module as an internal.

PR-URL: https://github.com/nodejs/node/pull/15759


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>