Add support for d:DesignInstance when using d:DataContext in designer

Error:

Cannot create unknown type '{http://schemas.microsoft.com/expression/blend/2008}DesignInstance'.

Repro:

<UserControl x:Class="Draw2D.Views.MainView"
             xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:editor="clr-namespace:Draw2D.Editor"
             mc:Ignorable="d"
             d:DataContext="{d:DesignInstance Type=editor:CanvasEditor, IsDesignTimeCreatable=True}"
             d:DesignWidth="800" d:DesignHeight="600">

[patrickklaeren]

Docs have moved to https://docs.avaloniaui.net/docs/getting-started/ide-support#design-time-properties FYI

[worldbeater]

For those who came across this issue while looking for a way to set a design-time DataContext. In Avalonia, this works as easy as:

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="clr-namespace:AssemblyName.DesignTime;assembly=AssemblyName"
             x:Class="AssemblyName.Views.LoginView">
    <Design.DataContext>
        <d:DesignTimeLoginViewModel />
    </Design.DataContext>
</UserControl>

See also: https://avaloniaui.net/docs/quickstart/vs-designer#design-time-properties

Should be quite easy to add support for d:DesignInstance, we just need to write a custom IXamlAstTransformer I guess.