Implement WebPage and provide compatibility with iOS 18.4's SwiftUI WebView

iOS 18.4 introduces a SwiftUI WebView component: https://developer.apple.com/documentation/webkit/webkit-for-swiftui

The good news is that the very simple configuration works out-of-the-box with SkipWeb, since they both have a WebView component that takes a URL initializer:

import SwiftUI
#if SKIP
import SkipWeb
#else
import WebKit
#endif

@available(iOS 18.4, *)
struct BrowserView : View {
    #if !SKIP
    @State var page = WebPage()
    #endif

    var body: some View {
        #if SKIP
        SkipWeb.WebView(url: URL(string: "https://skip.tools")!)
        #else
        WebKit.WebView(url: URL(string: "https://skip.tools")!)
        #endif

        // or simply:
        WebView(url: URL(string: "https://skip.tools")!)
    }
}

However, the WebPage API encompasses the functionality provided by SkipWeb's WebViewState, WebViewNavigator, and WebEngine. We might be able to create a new WebPage type that forwards the properties and functions to the right type. There might be some refactoring that will need to take place.

Start by reviewing Apple's WebPage API and the existing SkipWeb WebViewState, WebViewNavigator, and WebEngine types named in the issue. Determine which WebPage properties and functions need compatibility forwarding and whether WebView configuration requires refactoring. Done means SkipWeb supports the relevant iOS 18.4 WebPage and WebView usage without breaking existing behavior.