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

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

src/Vendor.cpp

@@ -0,0 +1,3 @@
+#include "Vendor.hpp"
+
+std::map<int32_t, std::vector<VendorListing>> Vendor::VendorTables;