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/Telemetry/TelemetryManager.cpp

@@ -2,9 +2,9 @@
 
 #include "MultiPlayerLocalPlayer.h"
 
-#include "..\Minecraft.World\LevelSettings.h"
-#include "..\Minecraft.World\LevelData.h"
-#include "..\Minecraft.World\Level.h"
+#include "../Minecraft.World/LevelSettings.h"
+#include "../Minecraft.World/LevelData.h"
+#include "../Minecraft.World/Level.h"
 
 #include "TelemetryManager.h"
 

Minecraft.Client/Common/Telemetry/TelemetryManager.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "..\..\Common\UI\UIEnums.h"
+#include "../../Common/UI/UIEnums.h"
 
 class CTelemetryManager
 {