process: add allowedNodeEnvironmentFlags property

`process.allowedNodeEnvironmentFlags` provides an API to validate and
list flags as specified in `NODE_OPTIONS` from user code.

Refs: https://github.com/nodejs/node/issues/17740


Signed-off-by: Christopher Hiller <boneskull@boneskull.com>

PR-URL: https://github.com/nodejs/node/pull/19335


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: John-David Dalton <john.david.dalton@gmail.com>
Reviewed-By: Sam Ruby <rubys@intertwingly.net>
Reviewed-By: Anna Henningsen <anna@addaleax.net>