input: Write a python function to perform a linear search on a list and return the index of the target element.
  assert linear_search([1, 2, 3, 4, 5], 3) == 2
  assert linear_search([10, 20, 30, 40, 50], 40) == 3
  assert linear_search([5, 10, 15, 20, 25], 100) == -1
output: def linear_search(lst, target):
      for index, element in enumerate(lst):
          if element == target:
              return index
      return -1