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

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

src