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/Trial/TrialMode.h

@@ -1,5 +1,5 @@
 #pragma once
-#include "..\Tutorial\FullTutorialMode.h"
+#include "../Tutorial/FullTutorialMode.h"
 
 class TrialMode : public FullTutorialMode
 {