Added prerequisites and a practice for the "darts" exercise (#2547)

* Added prerequisites for the "darts" exercise

* Added practice to the "darts" exercise

* Reverted "practices" so it's empty

Author: galabra

config.json

@@ -2225,7 +2225,12 @@
         "name": "Darts",
         "uuid": "6c64649b-ea81-4118-9e74-a0a55018ffbc",
         "practices": [],
-        "prerequisites": [],
+        "prerequisites": [
+          "arithmetic-operators",
+          "comparison",
+          "conditionals",
+          "numbers"
+        ],
         "difficulty": 3
       },
       {