input: Write a function to merge two lists into one.
  assert merge_lists([1, 2, 3], [4, 5, 6])==[1, 2, 3, 4, 5, 6]
  assert merge_lists(['a', 'b', 'c'], ['d', 'e', 'f'])==['a', 'b', 'c', 'd', 'e', 'f']
  assert merge_lists([], [1, 2, 3])==[1, 2, 3]
output: def merge_lists(list1, list2):
      return list1 + list2