input: Write a function to perform a linear search on a list and return the index of the target element.
  assert linear_search([7, -3, 10, 13, -6], -3) == 1
  assert linear_search([7, -3, 10, 13, -6], 10) == 2
  assert linear_search([1, 2, 3, 4, 5], 6) == -1
output: def linear_search(lst, target):
      for index, element in enumerate(lst):
          if element == target:
              return index
      return -1