[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/gm_commands/modifynpcstat.cpp

@@ -8,7 +8,7 @@ void command_modifynpcstat(Client *c, const Seperator *sep)
 		ListModifyNPCStatMap(c);
 		return;
 	}
-	
+
 	if (!c->GetTarget() || !c->GetTarget()->IsNPC()) {
 		c->Message(Chat::White, "You must target an NPC to use this command.");
 		return;
@@ -76,6 +76,7 @@ std::map<std::string, std::string> GetModifyNPCStatMap()
 		{ "hp_regen_per_second", "HP Regen Per Second" },
 		{ "int", "Intelligence" },
 		{ "_int", "Intelligence" },
+		{ "keeps_sold_items", "Keeps Sold Items" },
 		{ "level", "Level" },
 		{ "loottable_id", "Loottable ID" },
 		{ "mana_regen", "Mana Regen" },

zone/gm_commands/npcedit.cpp

@@ -1491,6 +1491,22 @@ void command_npcedit(Client *c, const Seperator *sep)
 			);
 			return;
 		}
+	} else if (!strcasecmp(sep->arg[1], "keeps_sold_items")) {
+		if (sep->IsNumber(2)) {
+			auto keeps_sold_items = static_cast<uint8_t>(std::stoul(sep->arg[2]));
+			n.keeps_sold_items = keeps_sold_items;
+			d = fmt::format(
+				"{} will {} Keep Sold Items.",
+				npc_id_string,
+				keeps_sold_items ? "now" : "no longer"
+			);
+		} else {
+			c->Message(
+				Chat::White,
+				"Usage: #npcedit keeps_sold_items [Flag] - Sets an NPC's Keeps Sold Items Flag [0 = False, 1 = True]"
+			);
+			return;
+		}
 	} else if (!strcasecmp(sep->arg[1], "setanimation")) {
 		if (sep->IsNumber(2)) {
 			auto animation_id   = std::stoul(sep->arg[2]);
@@ -1760,6 +1776,10 @@ void SendNPCEditSubCommands(Client *c)
 		Chat::White,
 		"Usage: #npcedit exp_mod [Modifier] - Sets an NPC's Experience Modifier [50 = 50%, 100 = 100%, 200 = 200%]"
 	);
+	c->Message(
+		Chat::White,
+		"Usage: #npcedit keeps_sold_items [Flag] - Sets an NPC's Keeps Sold Items Flag [0 = False, 1 = True]"
+	);
 	c->Message(
 		Chat::White,
 		"Usage: #npcedit setanimation [Animation ID] - Sets an NPC's Animation on Spawn (Stored in spawn2 table)"