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/Minecraft.h

@@ -38,9 +38,9 @@ class ColourTable;
 class MultiPlayerGameMode;
 class PsPlusUpsellWrapper;
 
-#include "..\Minecraft.World\File.h"
-#include "..\Minecraft.World\DisconnectPacket.h"
-#include "..\Minecraft.World\C4JThread.h"
+#include "../Minecraft.World/File.h"
+#include "../Minecraft.World/DisconnectPacket.h"
+#include "../Minecraft.World/C4JThread.h"
 #include "ResourceLocation.h"
 
 using namespace std;