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/Network/Sony/SQRNetworkManager.h

@@ -12,7 +12,7 @@
 
 #include <unordered_map>
 #if defined __PSVITA__
-#include "..\..\Minecraft.Client\PSVita\4JLibs\inc\4J_Profile.h"
+#include "../../Minecraft.Client/PSVita/4JLibs/inc/4J_Profile.h"
 #endif
 
 class SQRNetworkPlayer;