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/Xbox/Social/SocialManager.cpp

@@ -9,7 +9,7 @@
 #include "SocialManager.h"
 #include <assert.h>
 #include <xgraphics.h>
-#include "..\Sentient\SentientManager.h"
+#include "../Sentient/SentientManager.h"
 
 ///////////////////////////////////////////////////////////////////////////
 // Constants.