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

---------

Co-authored-by: Loki <lokirautio@gmail.com>

Minecraft.Client/Common/Tutorial/StatTask.cpp

@@ -1,8 +1,8 @@
 #include "stdafx.h"
-#include "..\..\Minecraft.h"
-#include "..\..\LocalPlayer.h"
-#include "..\..\StatsCounter.h"
-#include "..\..\..\Minecraft.World\net.minecraft.stats.h"
+#include "../../Minecraft.h"
+#include "../../LocalPlayer.h"
+#include "../../StatsCounter.h"
+#include "../../../Minecraft.World/net.minecraft.stats.h"
 #include "StatTask.h"
 
 StatTask::StatTask(Tutorial *tutorial, int descriptionId, bool enablePreCompletion, Stat *stat, int variance /*= 1*/)