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/Durango/Iggy/include/gdraw.h

@@ -6,7 +6,7 @@
 #ifndef __RAD_INCLUDE_GDRAW_H__
 #define __RAD_INCLUDE_GDRAW_H__
 
-#include "rrcore.h"
+#include "rrCore.h"
 
 #define IDOC
 

Minecraft.Client/Durango/Iggy/include/iggy.h

@@ -8,7 +8,7 @@
 #define IggyVersion "1.2.30"
 #define IggyFlashVersion "9,1,2,30"
 
-#include "rrcore.h"   // base data types, macros
+#include "rrCore.h"   // base data types, macros
 
 RADDEFSTART