src: expose MaybeInitializeContext to allow existing contexts

Splits the node.js specific tweak intialization of NewContext into a new
helper MaybeInitializeContext so that embedders with existing contexts
can still use them in a Node.js Environment now that primordials are
initialized and required so early.

Update MaybeInitializeContext to return MaybeLocal,

PR-URL: https://github.com/nodejs/node/pull/28544


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>