Switch away from the root WASDK package to the smaller WinUI package

As part of the Windows App SDK 1.8 changes, the team split up the package into several smaller ones. For example, there are:

• Microsoft.WindowsAppSDK.AI
• Microsoft.WindowsAppSDK.Base
• Microsoft.WindowsAppSDK.DWrite
• Microsoft.WindowsAppSDK.Foundation
• Microsoft.WindowsAppSDK.InteractiveExperiences
• Microsoft.WindowsAppSDK.ML
• Microsoft.WindowsAppSDK.Runtime
• Microsoft.WindowsAppSDK.Widgets
• Microsoft.WindowsAppSDK.WinUI

[!NOTE]
One observation is that the root WASDK has an = dependency, but the sub packages do not. This means that if a project references WASDK, the dependencies can't have different versions anywhere else.

This is a working issue and I will update as we find/discuss.

Pros (so far)

• Apps should be smaller as not everything is included
• Apps could use a different version of different parts of WASDK

Cons (so far)

• It is different to what we know works
• The ecosystem may not be ready
  • Win2D is using an older 1.6 version of WASDK, so if we use WinUI, we will end up with WASDK 1.6 AND WinUI 1.8+
  • Third Party libraries are similar in that they may be targeting MAUI and WASDK 1.7 but MAUI now would be WinUI 1.8 - making BOTH packages reach the app
• It may need evaluation to see what we should include by default to make most things work out the box
• The API will be reduced, so we may have customers expecting the AI/ML/Widgets APIs available.
• Are the components meant to work with a pick-and-choose versions?

Summary (so far)

This is a good change, but needs more discussion to fully ensure it is a non-breaking change.

Start by reviewing the current Windows App SDK package references and the compatibility questions listed in the issue, then compare them with the smaller WinUI package and related version constraints. Done means the project has a decided, non-breaking package strategy; this issue names no files or tests, so implementation entry points are not specified.