Added SkipWebUIDelegate as a cross-platform popup/window delegate (WebEngineConfiguration.uiDelegate) to control child-window creation and lifecycle. The delegate exposes:
webView(_:createWebViewWith:platformContext:) -> WebEngine? to allow/deny popup creation by returning a child engine or nil.
platformContext is platform-typed (WebKitCreateWindowParams on iOS, AndroidCreateWindowParams on Android) so apps can use native popup details while keeping one shared API.
webViewDidClose(_:child:) to notify when a child window is closed.
Soft-deprecated pageURL (String?) in favor of url (URL?) to more closely mirror the WKWebView