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/FireChargeItem.cpp

@@ -6,7 +6,7 @@
 #include "net.minecraft.world.h"
 #include "SoundTypes.h"
 #include "FireChargeItem.h"
-#include "tile.h"
+#include "Tile.h"
 
 FireChargeItem::FireChargeItem(int id) : Item(id)
 {