Support vanilla SwiftUI

This might not be possible at the moment, but right now you need a device with UIKit/AppKit as the SwiftUI views get converted to UI/NSViews immediately.

Start by tracing where SwiftUI views are converted to UIKit or AppKit and how snapshot tests are generated. Determine whether vanilla SwiftUI can be supported without requiring a device with UIKit or AppKit, then verify the behavior with an appropriate snapshot test.

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