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

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

Minecraft.Client/Common/UI/UIScene_ConnectingProgress.cpp

@@ -1,10 +1,10 @@
 #include "stdafx.h"
 #include "UI.h"
 #include "UIScene_ConnectingProgress.h"
-#include "..\..\Minecraft.h"
+#include "../../Minecraft.h"
 #ifdef _WINDOWS64
-#include "..\..\Windows64\Network\WinsockNetLayer.h"
-#include "..\..\..\Minecraft.World\DisconnectPacket.h"
+#include "../../Windows64/Network/WinsockNetLayer.h"
+#include "../../../Minecraft.World/DisconnectPacket.h"
 
 static int ConnectingProgress_OnRejectedDialogOK(LPVOID, int iPad, const C4JStorage::EMessageResult)
 {