input: Write a python function to find the number in the list closest to zero.
  assert closest_number([8, 5, -2, 10]) == -2
  assert closest_number([4, -1, -3, 2]) == -1
  assert closest_number([3, -2, 1, 0, 5]) == 0
  assert closest_number([10, -10, 5, -5]) == 5
output: def closest_number(lst):
      return min(lst, key=lambda x: (abs(x), x))