Start Marking Windows App SDK dim-down features

[agniuks]

Originally posted by @asklar in #9177 (comment):

We should start marking the dim-down features somehow.
[non blocking for this PR]
Here is one idea: have a FrameworkCapabilities API, which returns different things in UWP vs WinAppSDK vs. xaml islands, e.g.:

enum AppFrameworkKind {
  UWP,
  SystemXAMLIslands,
  WinAppSDK
}

enum FrameworkCapability {
  SuspendResume,
  // more
}
runtimeclass FrameworkCapabilities {
  static AppFrameworkKind AppFrameworkKind { get; }
  bool IsCapabilitySupported(Capability c);
}

This would then enable us to:

• autogenerate docs based on the framework capabilities
• allow native code (and JS) to query for the presence of these capabilities (e.g. like a DeviceInfo module API or similar)