input: Write a python function to append two lists together.
  assert append_lists([1, 2, 3], [4, 5, 6])==[1, 2, 3, 4, 5, 6]
  assert append_lists([10, 20], [30, 40, 50])==[10, 20, 30, 40, 50]
  assert append_lists([], [1, 2, 3])==[1, 2, 3]
output: def append_lists(list1, list2):
      return list1 + list2