This PR makes progress towards a more cohesive sample system by adding the infrastructure for sample options and viewing source code.

Overview:

• Adds XamlNamedPropertyRelay to enable accessing XAML declared x:Name properties from outside of a control.
  • This will help create more advanced option panes without cluttering up the code-behind shown to the user.
  • Added under a new project CommunityToolkit.Labs.Core.SourceGenerators.XamlNamedPropertyRelay. Separate as not all projects need this generator.
  • Example can be found in the second placeholder sample.
• Adds generated sample options which automatically generates properties to bind to and a sample page to control them.
  • Supports boolean and multiple choice
  • Example can be found in the first placeholder sample.
• Renames CommunityToolkit.Labs.Core to CommunityToolkit.Labs.Core.SourceGenerators, since it only contains generators and models that use them.
• Includes and displays source code for each sample. Assumes namespace matches folder structure.
• Auto-strips source generator attributes from the final compiled code. Prevents them from showing to the user as code-behind for the sample control.
• Adds source generator diagnostics for usage of attributes that belong to the Labs sample system.
• Adds unit tests for CommunityToolkit.Labs.Core.SourceGenerators, to ensure diagnostics are outputted correctly.

Demo: