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/Audio/SoundEngine.h

@@ -2,7 +2,7 @@
 class Mob;
 class Options;
 using namespace std;
-#include "..\..\Minecraft.World\SoundTypes.h"
+#include "../../Minecraft.World/SoundTypes.h"
 
 #include "miniaudio.h"