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