Add NodeJS implementation

[simolus3]

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.

[changeset-bot]

🦋 Changeset detected

Latest commit: 87179ff

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@powersync/node	Minor

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[stevensJourney]

I made some very minor comments. Overall this looks great to me.

[simolus3]

I ran the web tests locally, they still pass.