Write all the content of the array buffer (arr) to the writer (w).

  // Example writing to stdout
  const contentBytes = new TextEncoder().encode("Hello World");
  await Deno.writeAll(Deno.stdout, contentBytes);

  // Example writing to file
  const contentBytes = new TextEncoder().encode("Hello World");
  const file = await Deno.open('test.file', {write: true});
  await Deno.writeAll(file, contentBytes);
  Deno.close(file.rid);

  // Example writing to buffer
  const contentBytes = new TextEncoder().encode("Hello World");
  const writer = new Deno.Buffer();
  await Deno.writeAll(writer, contentBytes);
  console.log(writer.bytes().length);  // 11