Adjust folder structure of AppServices experiment

[Sergio0694]

Describe the bug

Note: partially blocked by CommunityToolkit/Tooling-Windows-Submodule#62.

This issue tracks fixing the folder structure of the AppServices experiment:

• The AppServices project should be under some subfolder under \src
• The source generator project should be under another \src folder
• The source generator unit tests should be under a centralized \tests subfolder

I mean ideally I'd imagine something like:

• src
  • CommunityToolkit.AppServices
  • CommunityToolkit.AppServices.SourceGenerator
• tests
  • CommunityToolkit.AppServices.Tests
  • CommunityToolkit.AppServices.SourceGenerator.Tests