Add support for design time discoverability of default style key.

**Feature request:** add support for design time discoverability of default style key.
**Goal:** VS XAML intellisense to support Copy Style/Template without running app. Mostly for platform controls like Button and 3rd party SDKs like Telerik.
**Details:** when creating WPF Core Custom Control Library project in VS, the project contains following:
themes/generic.xaml

MyControl.cs
public class MyControl : Control
{
static MyControl()
{
DefaultStyleKeyProperty.OverrideMetadata(typeof(MyControl), new FrameworkPropertyMetadata(typeof(MyControl)));
}
}

This is not design time friendly as we cannot find default style key via Roslyn INamedTypeSymbol for MyControl. One way to fix it is to add DefaultStyleKeyAttribute, e.g.
[DefaultStyleKey(typeof(MyControl))]
public class MyControl : Control

Note that VS project and item templates should be updated to DefaultStyleKeyAttribute.