input: Write a function to perform a linear search on a list and return the index of the element if found, or -1 if not found.
  assert linear_search([1, 5, 8, 3], 8) == 2
  assert linear_search([1, 5, 8, 3], 5) == 1
  assert linear_search([1, 5, 8, 3], 9) == -1
output: def linear_search(lst, target):
      for index, element in enumerate(lst):
          if element == target:
              return index
      return -1