process: clarify the pre- and post-condition of esm setup

This patch:

- Clarifies the dependency of the ESM loader initialization
  (`process.cwd()` and the value of `--loader`) in `node.js`.
- Moves the initialization of the per-isolate `importModuleDynamically`
  and `initializeImportMetaObject` callbacks into `node.js`
- Moves the initialization of the ESM loader into
  `prepareUserCodeExecution()` since it potentially involves
  execution of user code (similar to `--require` for CJS modules).

PR-URL: https://github.com/nodejs/node/pull/25530


Reviewed-By: Gus Caplan <me@gus.host>