SPIGOT-6357: Add World#getMinHeight

By: md_5 <git@md-5.net>

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

@@ -1505,6 +1505,15 @@ public interface World extends PluginMessageRecipient, Metadatable {
      */
     public double getHumidity(int x, int y, int z);
 
+    /**
+     * Gets the minimum height of this world.
+     * <p>
+     * If the min height is 0, there are only blocks from y=0.
+     *
+     * @return Minimum height of the world
+     */
+    public int getMinHeight();
+
     /**
      * Gets the maximum height of this world.
      * <p>