input: Write a python function to scale the features of an array between 0 and 1.
  assert feature_scale([1, 2, 3, 4, 5]) == [0.0, 0.25, 0.5, 0.75, 1.0]
  assert feature_scale([10, 20, 30, 40, 50]) == [0.0, 0.25, 0.5, 0.75, 1.0]
  assert feature_scale([-10, 0, 10, 20, 30]) == [0.0, 0.25, 0.5, 0.75, 1.0]
output: def feature_scale(arr):
      min_val = min(arr)
      max_val = max(arr)
      return [(x - min_val) / (max_val - min_val) for x in arr]