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/net.minecraft.world.item.h

@@ -34,7 +34,7 @@
 #include "ShearsItem.h"
 #include "ShovelItem.h"
 #include "SignItem.h"
-#include "SnowBallItem.h"
+#include "SnowballItem.h"
 #include "StoneSlabTileItem.h"
 #include "TileItem.h"
 #include "TilePlanterItem.h"