MarkdownTextBlock: Adds OnMarkdownParsed event

[jcoc611-microsoft]

Adds an event MarkdownTextBlock.OnMarkdownParsed, which is useful because consumers cannot otherwise know when the MarkdownDocument has finished being generated from the markdown text input. It also technically allows consumers to mutate the markdown parsed tree before the render step takes place.

[Arlodotexe]

Hitting these errors in CI:

D:\a\Labs-Windows\Labs-Windows\components\MarkdownTextBlock\src\MarkdownTextBlock.Events.cs(12,31): error CS0246: The type or namespace name 'LinkClickedEventArgs' could not be found (are you missing a using directive or an assembly reference?) [D:\a\Labs-Windows\Labs-Windows\components\MarkdownTextBlock\src\CommunityToolkit.WinUI.Controls.MarkdownTextBlock.csproj::TargetFramework=net9.0-windows10.0.26100.0]
[...]
D:\a\Labs-Windows\Labs-Windows\components\MarkdownTextBlock\src\MarkdownTextBlock.Events.cs(12,31): error CS0246: The type or namespace name 'LinkClickedEventArgs' could not be found (are you missing a using directive or an assembly reference?) [D:\a\Labs-Windows\Labs-Windows\components\MarkdownTextBlock\src\CommunityToolkit.WinUI.Controls.MarkdownTextBlock.csproj::TargetFramework=uap10.0.17763]

Run ./tooling/GenerateSingleSolution.ps1 -c MarkdownTextBlock -winui 2 from repo root to switch from Wasdk/WinUI3 to UWP/WinUI2 locally.