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']) == ['a', 'b', 'c', 'd']
  assert merge_lists([1, 2], [3, 4, 5]) == [1, 2, 3, 4, 5]
output: def merge_lists(list1, list2):
      return list1 + list2