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

---------

Co-authored-by: Loki <lokirautio@gmail.com>

Minecraft.Client/Common/XUI/XUI_Scene_Inventory_Creative.h

@@ -1,13 +1,13 @@
 #pragma once
 using namespace std;
 
-#include "..\Media\xuiscene_inventory_creative.h"
+#include "../Media/xuiscene_inventory_creative.h"
 #include "XUI_Scene_AbstractContainer.h"
 #include "XUI_CustomMessages.h"
-#include "..\..\..\Minecraft.World\AbstractContainerMenu.h"
-#include "..\..\..\Minecraft.World\SimpleContainer.h"
+#include "../../../Minecraft.World/AbstractContainerMenu.h"
+#include "../../../Minecraft.World/SimpleContainer.h"
 
-#include "..\UI\IUIScene_CreativeMenu.h"
+#include "../UI/IUIScene_CreativeMenu.h"
 
 #include <vector>