[WIP] wa-sqlite support

[rkistner]

This is experimental, and not quite ready yet. It does however demonstrate how wa-sqlite could be supported in principle, despite having a very different API from better-sqlite3 and node:sqlite.

Some remaining issues:

• Currently requires hacking & breaking existing tests, e.g. removing the logic to delete databases.
• Currently only supports an in-memory database.
• Error handling is not properly implemented yet - the tests report unhandled errors.
• This uses an outdated fork of wa-sqlite. It should be updated to wa-sqlite 1.x.

Additionally, adding worker management options would be great, and even better if it could re-use NodeJS worker_thread code.