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_EndPoem.cpp

@@ -2,9 +2,9 @@
 #include "UI.h"
 #include "UIScene_EndPoem.h"
 #include "UIBitmapFont.h"
-#include "..\..\Minecraft.h"
-#include "..\..\MultiplayerLocalPlayer.h"
-#include "..\..\..\Minecraft.World\StringHelpers.h"
+#include "../../Minecraft.h"
+#include "../../MultiPlayerLocalPlayer.h"
+#include "../../../Minecraft.World/StringHelpers.h"
 
 UIScene_EndPoem::UIScene_EndPoem(int iPad, void *initData, UILayer *parentLayer) : UIScene(iPad, parentLayer)
 {