input: Write a python function to sum two lists.
  assert list_sum([1, 2, 3], [4, 5, 6]) == 21
  assert list_sum([10, 20], [30, 40, 50]) == 150
  assert list_sum([0, 0, 0], [0, 0]) == 0
output: def list_sum(list1, list2):
      return sum(list1) + sum(list2)