Adds shoddy death match system.

It barely even works but oh well.

Content.Server/GameTicking/GameRule.cs

@@ -1,7 +1,18 @@
+using JetBrains.Annotations;
+
 namespace Content.Server.GameTicking
 {
-    public class GameRule
+    [PublicAPI]
+    public abstract class GameRule
     {
-        
+        public virtual void Added()
+        {
+
+        }
+
+        public virtual void Removed()
+        {
+
+        }
     }
 }