src: split the main context initialization from Environemnt ctor

So that it's possible to create an Environment not yet attached
to any V8 context. We'll use this to deserialize the V8 context
before attaching an Environment to it.

PR-URL: https://github.com/nodejs/node/pull/32984


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>