Add API to get a players experience to level (getExpToLevel). Implements BUKKIT-1906

This is the total experience one needs to gain a level.

By: feildmaster <admin@feildmaster.com>

paper-api/src/test/java/org/bukkit/plugin/messaging/TestPlayer.java

@@ -764,4 +764,8 @@ public class TestPlayer implements Player {
     public boolean isValid() {
         throw new UnsupportedOperationException("Not supported yet.");
     }
+
+    public int getExpToLevel() {
+        throw new UnsupportedOperationException("Not supported yet.");
+    }
 }