Added Difficulty API.

By: EvilSeph <evilseph@gmail.com>

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

@@ -725,6 +725,20 @@ public interface World {
      */
     public void setAutoSave(boolean value);
 
+    /**
+     * Sets the Difficulty of the world.
+     *
+     * @param difficulty the new difficulty you want to set the world to
+     */
+    public void setDifficulty(Difficulty difficulty);
+
+    /**
+     * Gets the Difficulty of the world.
+     *
+     * @return The difficulty of the world.
+     */
+    public Difficulty getDifficulty();
+
     /**
      * Represents various map environment types that a world may be
      */