input: Write a python function to append a number to a list.
  assert append_to_list([1, 4, 5, 8], 99) == [1, 4, 5, 8, 99]
  assert append_to_list([10, 20, 30], 40) == [10, 20, 30, 40]
  assert append_to_list([], 7) == [7]
output: def append_to_list(lst, num):
      lst.append(num)
      return lst