[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.

utils/sql/git/required/2022_12_24_npc_keeps_sold_items.sql

@@ -0,0 +1,2 @@
+ALTER TABLE `npc_types` 
+ADD COLUMN `keeps_sold_items` tinyint(1) UNSIGNED NOT NULL DEFAULT 1 AFTER `faction_amount`;