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

Author: addaleax

src/env.h

@@ -184,6 +184,7 @@ class ModuleWrap;
   V(kill_signal_string, "killSignal")                                         \
   V(length_string, "length")                                                  \
   V(mac_string, "mac")                                                        \
+  V(main_string, "main")                                                      \
   V(max_buffer_string, "maxBuffer")                                           \
   V(message_string, "message")                                                \
   V(minttl_string, "minttl")                                                  \