input: Write a python 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