Added the ability to retrieve a world's Seed.

By: Rigby <rigby@onarandombox.com>

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

@@ -489,6 +489,13 @@ public interface World {
         NETHER
     }
 
+    /**
+     * Gets the Seed for this world.
+     *
+     * @return This worlds Seed
+     */
+    public long getSeed();
+
     /**
      * Saves world to disk
      */