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/Durango/Leaderboards/DurangoLeaderboardManager.cpp

@@ -1,6 +1,6 @@
 #include "stdafx.h"
 #include "DurangoLeaderboardManager.h"
-#include "..\..\..\Minecraft.World\StringHelpers.h"
+#include "../../../Minecraft.World/StringHelpers.h"
 
 namespace WFC = Windows::Foundation::Collections;
 namespace CC = concurrency;

Minecraft.Client/Durango/Leaderboards/DurangoLeaderboardManager.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "Common\Leaderboards\LeaderboardManager.h"
+#include "Common/Leaderboards/LeaderboardManager.h"
 
 namespace P = Platform;
 namespace PC = Platform::Collections;

Minecraft.Client/Durango/Leaderboards/DurangoStatsDebugger.cpp

@@ -1,15 +1,15 @@
 #include "stdafx.h"
 
-#include "..\Minecraft.World\Tile.h"
-#include "..\Minecraft.World\Item.h"
+#include "../Minecraft.World/Tile.h"
+#include "../Minecraft.World/Item.h"
 
-#include "..\Minecraft.World\DurangoStats.h"
+#include "../Minecraft.World/DurangoStats.h"
 
-#include "..\Minecraft.World\EntityIO.h"
+#include "../Minecraft.World/EntityIO.h"
 
-#include "..\Minecraft.World\StringHelpers.h"
+#include "../Minecraft.World/StringHelpers.h"
 
-#include "Common\Console_Awards_enum.h"
+#include "Common/Console_Awards_enum.h"
 
 #include "DurangoStatsDebugger.h"
 

Minecraft.Client/Durango/Leaderboards/GameProgress.cpp

@@ -1,8 +1,8 @@
 #include "stdafx.h"
 
-#include "Durango\ServiceConfig\Events-XBLA.8-149E11AEEvents.h"
+#include "Durango/ServiceConfig/Events-XBLA.8-149E11AEEvents.h"
 
-#include "..\Minecraft.World\DurangoStats.h"
+#include "../Minecraft.World/DurangoStats.h"
 
 #include "GameProgress.h"