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/UI/UIScene_QuadrantSignin.cpp

@@ -1,9 +1,9 @@
 #include "stdafx.h"
 #include "UI.h"
 #include "UIScene_QuadrantSignin.h"
-#include "..\..\Minecraft.h"
+#include "../../Minecraft.h"
 #if defined(__ORBIS__)
-#include "Common\Network\Sony\SonyHttp.h"
+#include "Common/Network/Sony/SonyHttp.h"
 #endif
 
 UIScene_QuadrantSignin::UIScene_QuadrantSignin(int iPad, void *_initData, UILayer *parentLayer) : UIScene(iPad, parentLayer)