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/Common/UI/UIScene_SettingsGraphicsMenu.cpp

@@ -1,9 +1,9 @@
 #include "stdafx.h"
 #include "UI.h"
 #include "UIScene_SettingsGraphicsMenu.h"
-#include "..\..\Minecraft.h"
-#include "..\..\Options.h"
-#include "..\..\GameRenderer.h"
+#include "../../Minecraft.h"
+#include "../../Options.h"
+#include "../../GameRenderer.h"
 
 namespace
 {