chore: Adding Factory for empty and no-op States that can be used in SwiftUI's previews.

[ruisebas]

Issue #, if available:

• Xcode preview support for custom views #63

Description of changes:

This PR introduces a PreviewFactory that provides developers with empty and no-op States that can be used in Xcode's Previews when implementing customized views.

For example:

import Authenticator

struct CustomSignInView: View {
    @ObservedObject var state: SignInState

    var body: some View { /*...*/ }
}

#Preview {
    CustomSignInView(
        state: PreviewFactory.States.signIn()
    )
}

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.