process: allow StartExecution() to take a main script ID

The idea is to allow the C++ layer to run arbitrary scripts
as the main script. This paves the way for

- cctest of the execution of Node.js instances
- Earlier handling of per-process CLI options that affect
  execution modes (those usually do not make sense for the
  embedders).
- Targets like mkcodecache or mksnapshot.

Also moves the handling of `_third_party_main.js` into C++.

PR-URL: https://github.com/nodejs/node/pull/25474


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>