chore: add 2015D15 dataset

internal/data/2015/DayFifteen/input.txt

@@ -0,0 +1,4 @@
+Sprinkles: capacity 2, durability 0, flavor -2, texture 0, calories 3
+Butterscotch: capacity 0, durability 5, flavor -3, texture 0, calories 3
+Chocolate: capacity 0, durability 0, flavor 5, texture -1, calories 8
+Candy: capacity 0, durability -1, flavor 0, texture 5, calories 8