src: implement generic backend for process.env

Allow a generic string-based backing store, with no significance
to the remainder of the process, as a store for `process.env`.

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>