watchOS support

[importRyan]

Please add explicit watchOS support. While we sync auth state with iOS and macOS devices, we still need a fallback when iCloud Keychain is unavailable.

This SDK has guards such as:

@available(iOS 13.0, macOS 10.15, *)
private extension Image {
  static var signInButtonImage: Image {
   ...
#if os(iOS) || targetEnvironment(macCatalyst)
   ...
#elseif os(macOS)
   ...
#else
      fatalError("Unrecognized platform for SwiftUI sign in button image")
#endif