previews property
PreviewType -> the preview videos to publish, in the order Apple should
show them. Only types with a directory present appear at all.
Separate from screenshots rather than a fourth kind of entry in it,
because Apple keeps them in separate resources with separate enumerations
— appPreviewSets keyed by PreviewType, appScreenshotSets keyed by
ScreenshotDisplayType — and merging them here would mean a caller
deciding which endpoint a directory name belongs to, which is the guess
this whole model exists to remove.
Implementation
final Map<String, List<LocalPreview>> previews = {};