process: hide NodeEnvironmentFlagsSet's `add` function

This makes sure that the `add` function is not visible by default
when inspecting `process.allowedNodeEnvironmentFlags`.

PR-URL: https://github.com/nodejs/node/pull/28206


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>