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/UI/UIControl_EnchantmentBook.cpp

@@ -1,13 +1,13 @@
 #include "stdafx.h"
 #include "UI.h"
 #include "UIControl_EnchantmentBook.h"
-#include "..\..\Minecraft.h"
-#include "..\..\TileEntityRenderDispatcher.h"
-#include "..\..\EnchantTableRenderer.h"
-#include "..\..\Lighting.h"
-#include "..\..\BookModel.h"
-#include "..\..\..\Minecraft.World\net.minecraft.world.level.tile.entity.h"
-#include "..\..\..\Minecraft.World\net.minecraft.world.inventory.h"
+#include "../../Minecraft.h"
+#include "../../TileEntityRenderDispatcher.h"
+#include "../../EnchantTableRenderer.h"
+#include "../../Lighting.h"
+#include "../../BookModel.h"
+#include "../../../Minecraft.World/net.minecraft.world.level.tile.entity.h"
+#include "../../../Minecraft.World/net.minecraft.world.inventory.h"
 
 UIControl_EnchantmentBook::UIControl_EnchantmentBook()
 {