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/Common/GameRules/GameRuleDefinition.h

@@ -3,7 +3,7 @@ using namespace std;
 #include <unordered_map>
 #include <string>
 
-#include "..\..\..\Minecraft.World\ItemInstance.h"
+#include "../../../Minecraft.World/ItemInstance.h"
 #include "ConsoleGameRulesConstants.h"
 
 #include "GameRulesInstance.h"