src: declare process-related C++ methods in node_process.h

Instead of in node_internals.h. Also move process property
accessors that are not reused into node_process_object.cc
and make them static.

PR-URL: https://github.com/nodejs/node/pull/25397


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>