Run UserStoppedTypingBehavior Command on UIThread

[pictos]

Description of Change

Linked Issues

• Fixes [BUG] UserStoppedTypingBehavior Command does not run on the MainThread #2590

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information

[TheCodeTraveler]

Let's instead change void OnTextPropertyChanged to async void OnTextPropertyChanged and use async/await.

To be honest, I should've made this recommendation in the code review when this was originally implemented; the current implementation just attempts to recreate how Async/Await works under the hood, but does it in a less-efficient, less maintainable, manner than Async/Await.

[TheCodeTraveler]

Thanks Pedro!