Support external storage option?

[sindresorhus]

Sometimes you need to store something that is a bit larger, like a icon image. Storing this in UserDefaults would be slow as it has to be decoded each time it's accessed.

SwiftData has a .externalStorage option that makes it store it to a file on disk instead of in the database. Maybe we could do something similar.

static let icon = Key<NSImage?>("icon", externalStorage: true)

This can only be supported if the suite option is not given as otherwise we don't know where to store it.