Support for nullability annotations

**[Plan finalized - 3/28]**

Based on [community feedback](https://github.com/dotnet/wpf/discussions/7555#discussioncomment-5053915), here is the plan for supporting **Nullability annotations**.

List of assemblies and respective groups:

- Group 0
- [ ] System.Xaml
- [ ] System.Windows.Input.Manipulations
- Group 1
- [ ] WindowsBase
- Group 2
- [ ] System.Windows.Presentation
- [ ] DirectWriteForwarder
- [ ] UIAutomationTypes
- Group 3
- [ ] UIAutomationProvider
- [ ] PresentationCore
- Group 4
- [ ] ReachFramework
- [ ] UIAutomationClient
- Group 5
- [ ] System.Printing
- [ ] UIAutomationClientSideProviders
- [ ] PresentationFramework
- Group 6
- [ ] WindowsFormsIntegration
- [ ] PresentationUI
- Group 7
- [ ] Themes
- [ ] PresentationFramework.Aero
- [ ] PresentationFramework.Aero2
- [ ] PresentationFramework.AeroLite
- [ ] PresentationFramework.Classic
- [ ] PresentationFramework.Luna
- [ ] PresentationFramework.Royale
- Group 8
- [ ] PresentationFramework extensions
- [ ] SystemCore
- [ ] SystemData
- [ ] SystemDrawing
- [ ] SystemXml
- [ ] SystemXmlLinq
- [ ] System.Windows.Controls.Ribbon

Thanks @dotMorten for the [suggestion](https://github.com/dotnet/wpf/discussions/7555#discussioncomment-5072812)