major: Switch to forward slashes(+more) to fix compilation on Linux (#1403)

Notably also adds some metadata files for NixOS 

* add support for linux clang cross compiles

* add linux clang instructions

* un-capitalize Mob.horse.*

* update the description in flake.nix

---------

Co-authored-by: Loki <lokirautio@gmail.com>

Minecraft.Client/Common/Network/GameNetworkManager.h

@@ -2,21 +2,21 @@
 using namespace std;
 #include <vector>
 #include <qnet.h>
-#include "..\..\..\Minecraft.World\C4JThread.h"
+#include "../../../Minecraft.World/C4JThread.h"
 #include "NetworkPlayerInterface.h"
 #ifdef _XBOX
-#include "..\..\Xbox\Network\PlatformNetworkManagerXbox.h"
+#include "../../Xbox/Network/PlatformNetworkManagerXbox.h"
 #elif defined __PS3__ || defined __ORBIS__ || defined __PSVITA__
-#include "..\..\Common\Network\Sony\PlatformNetworkManagerSony.h"
+#include "../../Common/Network/Sony/PlatformNetworkManagerSony.h"
 #elif defined _DURANGO
-#include "..\..\Durango\Network\PlatformNetworkManagerDurango.h"
+#include "../../Durango/Network/PlatformNetworkManagerDurango.h"
 #else
 #include "PlatformNetworkManagerStub.h"
 #endif
 #include "SessionInfo.h"
 
 #ifdef __ORBIS__
-#include "..\..\Orbis\Network\PsPlusUpsellWrapper_Orbis.h"
+#include "../../Orbis/Network/PsPlusUpsellWrapper_Orbis.h"
 #endif
 
 class ClientConnection;