showOnboardingIfNeeded for custom onboarding flows

[JamesSedlacek]

I want to give people the ability to use their own custom onboarding flow with the view modifier.

import OnboardingKit
import SwiftUI

@main
struct ExampleApp: App {
    var body: some Scene {
        ContentView()
            .showOnboardingIfNeeded {
                CustomOnboardingFlow()
            }
    }
}