Add practice and prerequisites to "dnd-character" exercise (#2698)

* Update config.json

* Format config.json

* Update config.json

* Restore dnd character topics in config.json

Author: quintuple-mallard

config.json

@@ -2229,7 +2229,10 @@
         "name": "D&D Character",
         "uuid": "b373e13c-f179-4b36-b6e8-2a0f41540344",
         "practices": [],
-        "prerequisites": [],
+        "prerequisites": [
+          "randomness",
+          "classes"
+        ],
         "difficulty": 2,
         "topics": [
           "classes",