embedding: make `NewIsolate()` API more flexible

Split the API up into its essential parts, namely setting up
the creation parameters for the Isolate, creating it, and performing
Node.js-specific customization afterwards.

PR-URL: https://github.com/nodejs/node/pull/26525


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>