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/PSVita/Leaderboards/PSVitaLeaderboardManager.cpp

@@ -2,13 +2,13 @@
 
 #include "PSVitaLeaderboardManager.h"
 
-#include "PSVita\PSVita_App.h"
-#include "PSVita\PSVitaExtras\ShutdownManager.h"
+#include "PSVita/PSVita_App.h"
+#include "PSVita/PSVitaExtras/ShutdownManager.h"
 
-#include "Common\Consoles_App.h"
-#include "Common\Network\Sony\SQRNetworkManager.h"
+#include "Common/Consoles_App.h"
+#include "Common/Network/Sony/SQRNetworkManager.h"
 
-#include "..\..\..\Minecraft.World\StringHelpers.h"
+#include "../../../Minecraft.World/StringHelpers.h"
 
 #include <cstdlib>