Single project MSIX packaging

[daniel-lerch]

This pull request switches from an Windows Application Packaging project to single project MSIX packaging based on Microsoft.Windows.SDK.BuildTools.MSIX as discussed in microsoft/WindowsAppSDK#4932.

Pros:

• Library projects do not need explicit runtime identifiers anymore unlike with WAP: NETSDK1047 error using msbuild for WAP apps with project dependencies microsoft/WindowsAppSDK#3034
• No NuGet warnings anymore for referencing a .NET Core project from .NET Framework
• Possibility to package and debug with native AOT (not yet supported by WinForms)
• One project less in Visual Studio

Cons:

• No built in bundling anymore (GitHub Action & custom script)
• No documentation yet: Experimental single project MSIX packaging for arbitrary .NET projects MicrosoftDocs/msix-docs#441