src: add 'dynamic' process.release.lts property

This makes the process.release.lts property configurable by a constant.

This ref is the original PR to v6.x.
Refs: https://github.com/nodejs/node/pull/3212

 Conflicts:
        doc/api/process.md

PR-URL: https://github.com/nodejs/node/pull/16656


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>