This adds a NodeJS implementation based on the amazing work done by @spinda in this fork. I've followed up on that work by:

1. Offloading queries to workers, which allows concurrent readers.
2. Automatically downloading the prebuilt PowerSync core extension in an install script, and loading it when databases are opened (that seemed like the simplest option for now, bundling the shared libraries with the package would probably be a better approach).
3. Adding a small demo and beginning with a few simple tests.