esm: port loader code to JS

There is no reason for this to be in C++. Using JavaScript means that
the code is more accessible to more developers, which is important
for any Node.js feature. This also simplifies the code significantly
in some areas. On the technical side, this potentially also enables
making some of the file system operations that are involved
asynchronous.

PR-URL: https://github.com/nodejs/node/pull/32201


Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Guy Bedford <guybedford@gmail.com>