App Extension Info.plist Validation

[maustinstar]

This is a stacked PR dependent on #1944. To isolate the diffs from this branch, view here.

#1944 introduces support for App Extensions built with ExtensionKit by setting the attribute extensionkit_extension = True.

Issue #1943 discusses that we should catch and surface validation errors. This PR makes the extensionkit_extension attribute discoverable by providing warnings and errors with hints.

If extensionkit_extension = False (default), and we recognize an extension point identifier is intended for ExtensionKit (like com.apple.app-intents), provide a warning, and suggest setting extensionkit_extension = True. These warnings are non-blocking, since Apple may change extension type support for products.

Likewise, if extensionkit_extension = False but the Info.plist contains an EXAppExtensionAttributes entry, raise a blocking error. app-extension product types always expect the NSExtension entry, and never EXAppExtensionAttributes. Conversely, extensionkit-extension product types always expect EXAppExtensionAttributes entry, and never NSExtension.