input: Write a function to sum all elements in a nested list.
  assert sum_nested_list([1, [2, [3, 4], 5], 6]) == 21
  assert sum_nested_list([1, [2, 3], [4, 5], 6]) == 21
  assert sum_nested_list([1, [2, [3, [4, 5]]], 6]) == 21
output: def sum_nested_list(lst):
      total = 0
      for element in lst:
          if isinstance(element, list):
              total += sum_nested_list(element)
          else:
              total += element
      return total