Pass all SessionDelegate callbacks through to TurboNavigationDelegate

[joemasilotti]

Continuing the conversation from #72, this approach passes all unused SessionDelegate callbacks through TurboNavigationDelegate without modification.

This gives developers an option to override whatever part of SessionDelegate they want. They can implement their functionality in their existing TurboNavigationDelegate extension.

extension SceneDelegate: TurboNavigationDelegate {
    func sessionDidFinishRequest(_ session: Session) {
        // TODO: Custom handling.
    }
}

It also provides a default WKUIDelegate implementation, handling alerts and confirmation dialogs. If a developer wants more customization they are encouraged to copy-paste the extension TurboNavigator: WKUIDelegate extension and overwrite and/or add functionality of their own. Making currentNavigationController public enables this to be possible.

[joemasilotti]

Closed in favor of #74.