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/PS3/SPU_Tasks/ChunkUpdate/Tesselator_SPU.h

@@ -6,7 +6,7 @@ typedef unsigned int DWORD;
 typedef unsigned char byte;
 #include <float.h>
 #else 
-#include "..\..\..\stdafx.h"
+#include "../../../stdafx.h"
 class ChunkRebuildData;
 #endif
 #endif