Expand the node-test.js example code with documentation

[jelmervdl]

This looked like the easiest way to provide some documentation & examples for the WASM API.

I've upgraded to node 18 because that allows us to use fetch() which matches what you would use in the browser.

Every line of code is annotated with some documentation on why it is there and what you can do with it.

[XapaJIaMnu]

What about this one @jelmervdl

[jelmervdl]

We can merge this as well. It works as documentation for the lower level API (the one exposed directly by the WASM interop code).