Enable extension loading in node

[carlopi]

Still using the patch, to be moved to external component but I am trying to find the proper place for the refactor.

Idea is this:

• if you are on a browser, you have XMLHttpRequest, use that to perform requests.
• if you are on node, and the file is available locally, use that
• otherwise, spawn a Worker, and via a combination of postMessage and Atomics, have the worker use async fetch made that will appear synchronous to the C++ code due to the wait

This makes node test passing, and it's the last blocker for tagging a version on npm.