refactor(Core/Entities): Cleanup Unit header file (#19622)

* move several enum definitions in a new files

* move more things

* remove the include comment

* move the related code to CharmInfo in this own file

*original idea of Ovahlord

Co-Authored-By: Ovahlord <18347559+Ovahlord@users.noreply.github.com>

* fix clang build errors and few blank lines

* missing forward declaration

* new tentative to fix gcc/clang build

* add several include could be necessary for gcc/clang compilation

* .

* revert some include since the last AC update

* fix some loop inconsistencies

* align all enums in UnitDefine.h

---------

Co-authored-by: Ovahlord <18347559+Ovahlord@users.noreply.github.com>

src/server/game/Entities/Item/Item.h

@@ -35,6 +35,12 @@ struct ItemSetEffect
     SpellInfo const* spells[8];
 };
 
+enum InventorySlot
+{
+    NULL_BAG    = 0,
+    NULL_SLOT   = 255
+};
+
 // EnumUtils: DESCRIBE THIS
 enum InventoryResult : uint8
 {