src: improve handling of internal field counting

Change suggested by bnoordhuis.

Improve handing of internal field counting by using enums.
Helps protect against future possible breakage if field
indexes are ever changed or added to.

Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/31960


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>