input: Write a function that takes a positive integer as input and uses a while loop to output the numbers counting down to 1, followed by a message "Done!".
  assert countdown(6) == "6\n5\n4\n3\n2\n1\nDone!"
  assert countdown(3) == "3\n2\n1\nDone!"
  assert countdown(1) == "1\nDone!"
  assert countdown(0) == "Done!"
output: def countdown(n):
      result = ""
      while n > 0:
          result += str(n) + "\n"
          n -= 1
      result += "Done!"
      return result