refactor(lua): ItemUtil.lua is Item.lua

The file holds retail's Item mixin (Blizzard_ObjectAPI/Mainline/Item.lua)
and its ItemEventListener, not retail's ItemUtil, so it takes Item.lua's
name. No code changes.

src/embed/AddOns/!!!ClassicAPI/!!!ClassicAPI.toc

@@ -41,7 +41,7 @@ Util\ColorUtil.lua
 Util\ItemLocation.lua
 Util\PlayerLocation.lua
 Util\EventUtil.lua
-Util\ItemUtil.lua
+Util\Item.lua
 Util\AddOnCompat.lua
 Util\FunctionUtil.lua
 Util\CacheUtil.lua

src/item/Exists.cpp

@@ -42,7 +42,7 @@ static int __fastcall Script_C_Item_DoesItemExist(void *L) {
 // optimistically accept any positive itemID; callers needing the
 // "is the cache populated?" check should use `IsItemDataCachedByID`.
 //
-// This split matters for `ItemMixin:IsItemEmpty` (FrameXML ItemUtil),
+// This split matters for `ItemMixin:IsItemEmpty` (Util/Item.lua),
 // which uses `DoesItemExistByID` to gate `ContinueOnItemLoad` — the
 // whole point of `ContinueOnItemLoad` is to wait for an uncached item
 // to load, so the existence check must succeed for items that aren't