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/Orbis/Leaderboards/OrbisLeaderboardManager.cpp

@@ -4,19 +4,19 @@
 
 #include "base64.h"
 
-#include "..\Orbis_App.h"
-#include "..\..\Common\Consoles_App.h"
+#include "../Orbis_App.h"
+#include "../../Common/Consoles_App.h"
 
-#include "Common\Network\Sony\SQRNetworkManager.h"
+#include "Common/Network/Sony/SQRNetworkManager.h"
 
-#include "..\..\..\Minecraft.World\StringHelpers.h"
+#include "../../../Minecraft.World/StringHelpers.h"
 
 #include <cstdlib>
 
 #include <np.h>
 //#include <sys/ppu_thread.h>
 
-#include "Orbis\OrbisExtras\ShutdownManager.h"
+#include "Orbis/OrbisExtras/ShutdownManager.h"
 
 
 LeaderboardManager *LeaderboardManager::m_instance = new OrbisLeaderboardManager(); //Singleton instance of the LeaderboardManager

Minecraft.Client/Orbis/Leaderboards/OrbisLeaderboardManager.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "Common\Leaderboards\LeaderboardManager.h"
+#include "Common/Leaderboards/LeaderboardManager.h"
 
 class OrbisLeaderboardManager : public LeaderboardManager
 {