src: improve module loader readability

Various improvements on readability, performance and
conformity to the Node core coding style in the ESM loader C++ code:

- `isolate` for the `Isolate*`, `context` for the `Local<Context>`
- Less reliance on `auto` where it’s unnecessary/increases cognitive
  overhead
- Shorter paths to failure via `.ToLocal()` & co
- Do not keep pending exceptions around e.g. for failed `JSON` parsing
- Use `Maybe` types to get explicit error status forwarding
- Remove an unnecessary handle scope
- Add `nullptr` checks for unwrapped host objects
- Remove unused code
- Use `CamelCase` for function names
- Use `const Foo&` instead of copying values whenever possible
- Pass along the `Environment*` explicitly

PR-URL: https://github.com/nodejs/node/pull/16536


Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>