[Feature] Update Designer Metadata to new VS2019 Model

[michael-hawker]

Describe the problem this feature would solve

In VS 2019 they changed the model on how the designer loads metadata for components. See article here: https://github.com/microsoft/xaml-designer-extensibility/blob/master/documents/xaml-designer-extensibility-migration.md

The toolkit should update our designer metadata assemblies to align to this new pattern.

Hello, 'michael-hawker! Thanks for submitting a new feature request. I've automatically added a vote 👍 reaction to help get things started. Other community members can vote to help us prioritize this feature in the future!

[michael-hawker]

Just realizing the new C# source generator features could help with this in the future, no?

[michael-hawker]

@Nirmal4G has volunteered to take a look at this initial investigation, thank you so much for taking a look, let us know what you find! 🦙❤

Let us know if this looks straight-forward.

(FYI, our code-complete for 6.1 is May 21st; however, I think this would constitute a breaking change, even though it's been long-overdue. So, we'd probably just delegate it to the 7.0 release later this summer when making any required changes can be done without problems.)

[Nirmal4G]

I'm working on it, but I don't know how l was unassigned. Somehow deleting the previous comment removed my assignment.