make data-server usable as a websocket library

[mingfang]

Currently the data-server websocker standalone server is standalone.
Sometimes that's not desirable because you already have your own server, e.g. Express.
To make data-server usable in Express, we need to export queryHandler and socketResponse.

[mingfang]

Here is an example use case with Express

import express from 'express'
import expressWebsockets from "express-ws"
import { DuckDB, Cache, queryHandler, socketResponse } from "@uwdata/mosaic-duckdb"

const { app } = expressWebsockets(express())
const db = new DuckDB()
const CACHE_DIR = '.mosaic/cache'
const queryCache = new Cache({ dir: CACHE_DIR })
const handleQuery = queryHandler(db, queryCache)
app.ws("/duckdb-ws", (websocket, request) => {
    const res = socketResponse(websocket)
    websocket.on('message', data => handleQuery(res, data))
})