Establish Design Process for WPF APIs and XAML

# Establish Design Process for WPF APIs and XAML

WPF exposes a rich set of APIs and also the Xaml syntax. We need to establish an open design process, both for additions in general and to accept and shape community contributions. This is very similar to [dotnet/apireviews](https://github.com/dotnet/apireviews) and [dotnet/csharplang](https://github.com/dotnet/csharplang). Those teams have established effective design processes that we can adopt for WPF. For APIs, we should just use the existing .NET API review process. For XAML, we'll need do something different. We need the WPF team to define these processes.

For folks filing issues that propose new API or Xaml syntax, please reference this issue as a way of upvoting its importance.