input: Write a python function to add two lists element-wise.
  assert list_add([1, 2, 3], [4, 5, 6]) == [5, 7, 9]
  assert list_add([10, 20, 30], [1, 2, 3]) == [11, 22, 33]
  assert list_add([0, 0, 0], [1, 1, 1]) == [1, 1, 1]
output: def list_add(list1, list2):
      return [a + b for a, b in zip(list1, list2)]