onPreferenceChange

[dfabulich]

Fixes #133

This PR is weird in several ways.

First, it appears that SKIP DECLARE won't let me disregard the complex where clause of the declaration of this function. If I uncomment the K.Value : Equatable part, Skip will complain, "Skip does not support the referenced type as a generic constraint." I would have expected SKIP DECLARE to tell Skip to disregard that problem, but it doesn't. (Relaxing that constraint should be harmless in practice, because the Xcode build will reject it if someone tries to make a PreferenceKey of a non-Equatable value.) I've filed this as a bug on the transpiler. skiptools/skip#432

Second, I'm using snapshotFlow; I have no idea how I'd port that to Swift, so I just didn't; I'm heavily using SKIP INSERT for this.

Third, you can't just build out an ordinary PreferenceKey and use it with preference. I had to declare my PreferenceKey like this:

struct TestPreferenceKey: PreferenceKey {
    typealias Value = String
    
    #if SKIP
    // SKIP DECLARE: companion object: PreferenceKeyCompanion<String>
    final class Companion: PreferenceKeyCompanion {
        let defaultValue = ""
        func reduce(value: inout String, nextValue: () -> String) {
            value = nextValue()
        }
    }
    #else
    static var defaultValue: String = ""

    static func reduce(value: inout String, nextValue: () -> String) {
        value = nextValue()
    }
    #endif
}

That is, in SwiftUI, I could just declare a couple of static methods, but in SkipUI, I had to explicitly declare a companion object so I could declare it a PreferenceKeyCompanion.

This is all quite strange, but it does work. (And isn't that really what Skip is all about? 😉)

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