Update ExpCommand with levels support. Fixes BUKKIT-2683 and partially fixes BUKKIT-2671

By: EvilSeph <evilseph@gmail.com>

paper-api/src/main/java/org/bukkit/entity/Player.java

@@ -370,6 +370,13 @@ public interface Player extends HumanEntity, Conversable, CommandSender, Offline
      */
     public void giveExp(int amount);
 
+    /**
+     * Gives the player the amount of experience levels specified. Levels can be taken by specifying a negative amount.
+     *
+     * @param amount amount of experience levels to give or take
+     */
+    public void giveExpLevels(int amount);
+
     /**
      * Gets the players current experience points towards the next level.
      * <p />