Add an example with a web worker

[Fil]

  const importScript = u =>
    window.location.href.includes(".observableusercontent.com")
      ? `https://unpkg.com/${u}`
      : `${window.location.href.replace(/\/[^/]*$/, "")}/${u}.js`;

  const worker = createWorkerFrom(`
  // load external scripts
  importScripts("${importScript("d3@5")}");
  importScripts("${importScript("d3-delaunay@5")}");
  …

`)