Broadcast a message on IO pub channel.

await Deno.jupyter.broadcast("display_data", {
  data: { "text/html": "<b>Processing.</b>" },
  metadata: {},
  transient: { display_id: "progress" }
});

await new Promise((resolve) => setTimeout(resolve, 500));

await Deno.jupyter.broadcast("update_display_data", {
  data: { "text/html": "<b>Processing..</b>" },
  metadata: {},
  transient: { display_id: "progress" }
});