input: Write a python function to perform a linear search on a list.
  assert linear_search([1, 2, 3, 4, 5], 3) == 2
  assert linear_search([10, 20, 30, 40, 50], 25) == None
  assert linear_search([7, 14, 21, 28], 28) == 3
output: def linear_search(lst, target):
      for index, value in enumerate(lst):
          if value == target:
              return index
      return None