input: Write a 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], 10) == 0
  assert linear_search([5, 8, 12, 20, 25], 15) == -1
output: def linear_search(arr, target):
      for index, value in enumerate(arr):
          if value == target:
              return index
      return -1