[refactor] Remove the word 'Manager' from most source files/namespaces

ChatManager -> Chat
MissionManager -> Missions
NanoManager -> Nanos
TransportManager -> Transport
ChunkManager -> Chunking
BuddyManager -> Buddies
GroupManager -> Groups
RacingManager -> Racing
ItemManager -> Items

NPCManager and PlayerManager remain.

Note: You can use git log --follow src/file.cpp to trace the history of
a file from before it was renamed.

src/Trading.hpp

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "ItemManager.hpp"
+#include "Items.hpp"
 
 namespace Trading {
     void init();