Add API for managing mob spawn limits per-world. Fixes BUKKIT-1565

By: Travis Watkins <amaranth@ubuntu.com>

paper-api/src/main/java/org/bukkit/Bukkit.java

@@ -353,4 +353,16 @@ public final class Bukkit {
     public static HelpMap getHelpMap() {
         return server.getHelpMap();
     }
+
+    public static int getMonsterSpawnLimit() {
+        return server.getMonsterSpawnLimit();
+    }
+
+    public static int getAnimalSpawnLimit() {
+        return server.getAnimalSpawnLimit();
+    }
+
+    public static int getWaterAnimalSpawnLimit() {
+        return server.getWaterAnimalSpawnLimit();
+    }
 }