src: allow per-Environment set of env vars

Abstract the `process.env` backing mechanism in C++ to allow
different kinds of backing stores for `process.env` for different
Environments.

PR-URL: https://github.com/nodejs/node/pull/26544
Fixes: https://github.com/nodejs/node/issues/24947


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>