src: provide a variant of LoadEnvironment taking a callback

This allows embedders to flexibly control how they start JS code
rather than using `third_party_main`.

PR-URL: https://github.com/nodejs/node/pull/30467


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>