[Feature] Add "Keeps Sold Items" Flag to NPCs (#2671)

# Perl
- Add `$npc->GetKeepsSoldItems()`.
- Add `$npc->SetKeepsSoldItems(keeps_sold_items)`.

# Lua
- Add `npc:GetKeepsSoldItems()`.
- Add `npc:SetKeepsSoldItems(keeps_sold_items)`.

# Notes
- Allows operators to keep specific NPCs from keeping items sold to them.
- Keeps NPCs from being cluttered with stuff like Cloth Caps, Bone Chips, etc.

zone/lua_npc.h

@@ -159,6 +159,8 @@ public:
 	float GetNPCStat(std::string stat);
 	void SendPayload(int payload_id);
 	void SendPayload(int payload_id, std::string payload_value);
+	bool GetKeepsSoldItems();
+	void SetKeepsSoldItems(bool keeps_sold_items);
 };
 
 #endif