Check if a given resource id (rid) is a TTY.

  // This example is system and context specific
  const nonTTYRid = Deno.openSync("my_file.txt").rid;
  const ttyRid = Deno.openSync("/dev/tty6").rid;
  console.log(Deno.isatty(nonTTYRid)); // false
  console.log(Deno.isatty(ttyRid)); // true
  Deno.close(nonTTYRid);
  Deno.close(ttyRid);