InventoryView QOL open method (#12282)

* Add QOL open method to InventoryView

* Check to ensure the opening isn't a InventoryMenu, allow HorseMenus

* Fix instanceof against API instaed of AbstractContainerMenu

* [ci skip] Remove suggested comment

paper-api/src/main/java/org/bukkit/inventory/InventoryView.java

@@ -242,6 +242,11 @@ public interface InventoryView {
     @NotNull
     public InventoryType.SlotType getSlotType(int slot);
 
+    /**
+     * Opens the inventory view.
+     */
+    void open();
+
     /**
      * Closes the inventory view.
      */