input: Write a function to perform a linear search in an array.
  assert linear_search([1, 2, 3, 4, 5], 3) == 2
  assert linear_search([10, 20, 30, 40, 50], 25) == -1
  assert linear_search(['a', 'b', 'c', 'd'], 'c') == 2
  assert linear_search(['apple', 'banana', 'cherry'], 'banana') == 1
output: def linear_search(arr, target):
      for i, element in enumerate(arr):
          if element == target:
              return i
      return -1