src: use enum for refed flag on native immediates

Refs: https://github.com/nodejs/node/pull/33320#discussion_r423141443

PR-URL: https://github.com/nodejs/node/pull/33444


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>