input: Write a python function to append an item to a list.
  assert append_to_list([1, 2, 3], 4) == [1, 2, 3, 4]
  assert append_to_list(['a', 'b'], 'c') == ['a', 'b', 'c']
  assert append_to_list([], 'new_item') == ['new_item']
output: def append_to_list(lst, item):
      lst.append(item)
      return lst