Add Observable client to duckdb-wasm

[domoritz]

Move the client from https://observablehq.com/@cmudig/duckdb here and add tests etc.

[domoritz]

Already started at observablehq/stdlib#313 (comment)

[domoritz]

Done. The client is now in Observable's standard library. See https://github.com/observablehq/stdlib/blob/main/src/duckdb.js