n-api: name CallbackBundle function fields

Use field names rather than indices.

Refs: https://github.com/nodejs/node/pull/21072

PR-URL: https://github.com/nodejs/node/pull/21240


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>