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/GameRenderer.h

@@ -14,8 +14,8 @@ class SparseLightStorage;
 class CompressedTileStorage;
 class SparseDataStorage;
 
-#include "..\Minecraft.World\SmoothFloat.h"
-#include "..\Minecraft.World\C4JThread.h"
+#include "../Minecraft.World/SmoothFloat.h"
+#include "../Minecraft.World/C4JThread.h"
 #include "ResourceLocation.h"
 
 class GameRenderer