Add @CustomDump macro

[stephencelis]

Filling a similar niche as Swift's upcoming @DebugDescription macro, it will automatically generate a CustomDumpRepresentable conformance for a class with an inner, equatable CustomDumpValue. This can be used for better exhaustive testing via expectDifference:

@CustomDump
@MainActor
@Observable
final class FeatureModel {
  var count = 0

  func incrementButtonTapped() { count += 1 }
}

let model = FeatureModel()
expectDifference(model) {
  model.incrementButtonTapped()
} changes: {
  $0.count = 1
}