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

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