[refactor] Split ItemManager.cpp into Vendor.cpp & Trading.cpp

- added sources to Makefile
- Added Trading::init() to main.cpp

src/Vendor.hpp

@@ -0,0 +1,11 @@
+#pragma once
+
+#include "ItemManager.hpp"
+
+struct VendorListing {
+    int sort, type, iID;
+};
+
+namespace Vendor {
+    extern std::map<int32_t, std::vector<VendorListing>> VendorTables;
+}