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/PS3/PS3Extras/DirectX/sal.h

@@ -157,7 +157,7 @@
 #endif
 
 #if defined(_MSC_EXTENSIONS) && !defined( MIDL_PASS ) && !defined(__midl) && !defined(RC_INVOKED)
-#include "codeanalysis\sourceannotations.h"
+#include "codeanalysis/sourceannotations.h"
 #endif
 
 //============================================================================