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/XUI/XUI_SkinSelect.h

@@ -2,8 +2,8 @@
 
 #include "../media/xuiscene_skinselect.h"
 #include "XUI_CustomMessages.h"
-#include "..\..\..\Minecraft.World\Definitions.h"
-#include "..\..\Textures.h"
+#include "../../../Minecraft.World/Definitions.h"
+#include "../../Textures.h"
 
 class DLCPack;