input: Write a function to create a Python program that takes in a positive integer, then uses a while loop to output the numbers counting down to 1.
  
  assert countdown_to_one(6) == "6\n5\n4\n3\n2\n1\nDone!"
  assert countdown_to_one(3) == "3\n2\n1\nDone!"
  assert countdown_to_one(1) == "1\nDone!"
  assert countdown_to_one(10) == "10\n9\n8\n7\n6\n5\n4\n3\n2\n1\nDone!"
  assert countdown_to_one(0) == "Done!"
output: def countdown_to_one(n):
      result = ""
      while n > 0:
          result += f"{n}\n"
          n -= 1
      result += "Done!"
      return result