src: split up InitializeContext

This splits out code from InitializeContext into a
new function InitializeContextForSnapshot and
moves the callsite of InitializeContextRuntime from
NewContext to InitializeContext - embedders don't
necessarily call NewContext and so need to be able
to guarantee these functions are called regardless.

PR-URL: https://github.com/nodejs/node/pull/30067


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>