message: added auto-open of demo url
diff: * REQS: express
  * USAGE: `node browser_server.mjs`
  * AUTH: Brent Ely (https://github.com/gitbrent/)
  - * DATE: 20210404
  + * DATE: 20210602
  */
  +// Use `createRequire` as `require` wont work by default in modules
  +import { createRequire } from "module";
  +const require = createRequire(import.meta.url);
  +
  import express from "express";
  const app = express();
  const port = 8000;
  +const DEMO_URL = `http://localhost:${port}/browser/index.html`;
  app.use("/browser", express.static("./browser"));
  app.use("/common", express.static("./common"));
  @@ -18,7 +23,10 @@ app.use("/modules", express.static("./modules"));
  app.listen(port, () => {
  console.log(`\n----------------------==~==~==~==[ SERVER RUNNING ]==~==~==~==----------------------\n`);
  - console.log(`The pptxgenjs browser demo is now live at: http://localhost:${port}/browser/index.html\n`);
  + console.log(`The pptxgenjs browser demo is now live at: ${DEMO_URL}\n`);
  console.log(`(Press Ctrl-C to stop)`);
  console.log(`\n----------------------==~==~==~==[ SERVER RUNNING ]==~==~==~==----------------------\n`);
  });
  +
  +let start = process.platform == "darwin" ? "open" : process.platform == "win32" ? "start" : "xdg-open";
  +require("child_process").exec(start + " " + DEMO_URL);
  