input: Write a python function to perform a linear search for a number in a list and return its index.
  assert linear_search([1, 2, 3, 4, 5], 3) == 2
  assert linear_search([10, 20, 30, 40, 50], 20) == 1
  assert linear_search([5, 10, 15, 20, 25], 25) == 4
  assert linear_search([7, 14, 21, 28], 14) == 1
  assert linear_search([1, 2, 3, 4, 5], 6) == -1
output: def linear_search(lst, target):
      for index, value in enumerate(lst):
          if value == target:
              return index
      return -1