Authoring support for unsealed types

[angelazhangmsft]

Proposal: Support authoring unsealed types

Rationale

Currently WinRT only allows authoring sealed types. Adding unsealed type support will help adopters such as Windows Community Toolkit, Notifications component, and those who use inheritance. Authors otherwise may use class libraries to work around limitations with sealed classes.

[asklar]

Count me as very interested in this :)

[asklar]

This will prevent defining extensible (inheritable) WinUI 3 controls, can this please be prioritized?