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.World/MobSpawner.h

@@ -3,7 +3,7 @@
 #include "Mob.h"
 
 #ifdef __PSVITA__
-#include "..\Minecraft.Client\PSVita\PSVitaExtras\CustomMap.h"
+#include "../Minecraft.Client/PSVita/PSVitaExtras/CustomMap.h"
 #endif
 
 class Player;