Scenario: analyzers\cs (for roslyn source generators, for example).
This would mean it's a custom (non-TFM specific) folder/PackageItemKind.
In this sense, PackageItemKind is basically a shorthand for PackageFolder+FrameworkSpecific really.

I don't think we need PackageFolder + FrameworkSpecific project property though. The framework-specific folders are well-known and built-in, so I don't foresee a need for that combination.