Added world.getSeaLevel()

By: Dinnerbone <dinnerbone@dinnerbone.com>

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

@@ -686,6 +686,15 @@ public interface World {
      */
     public int getMaxHeight();
 
+    /**
+     * Gets the sea level for this world.
+     *
+     * This is often half of {@link #getMaxHeight()}
+     *
+     * @return Sea level
+     */
+    public int getSeaLevel();
+
     /**
      * Gets whether the world's spawn area should be kept loaded into memory or not.
      *