Support web-standards platforms for the data server

[tmcw]

It'd be nice to be able to run the mosaic data server on Cloudflare Workers or Deno, and it seems kind of feasible from how it's set up currently - the only big change would be the native Node.js duckdb module, which would have to be swapped for the WASM port.

[jheer]

This sounds exciting, though admittedly I have little-to-no experience with either. The WASM port has notable scalability limits (e.g., file system access, parallelization), so it would be ideal if a more full-featured DuckDB instance could be used. In addition to the Node-based data server, it should not be too difficult to write an analogous Python server (which is basically what the Jupyter widget does), should that be easier to deploy.