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/PS3/Leaderboards/PS3LeaderboardManager.h

@@ -1,9 +1,9 @@
 #pragma once
 
-#include "..\..\Common\Leaderboards\LeaderboardManager.h"
-#include "..\..\..\Minecraft.World\x64headers\extraX64.h"
+#include "../../Common/Leaderboards/LeaderboardManager.h"
+#include "../../../Minecraft.World/x64headers/extraX64.h"
 
-#include "PS3\Passphrase\ps3__np_conf.h"
+#include "PS3/Passphrase/ps3__np_conf.h"
 
 using namespace std;