Remove configuration in favor of delegate

[joemasilotti]

This PR builds on #73, so the diff showing is bigger than it "actually" is.

The changes move the configuration of the user agent and web view instantiation to the TurboNavigationDelegate protocol. This makes customizing the web view a bit cleaner when integrating with Strada. And I found that a lot of folks were confused by the block used previously, so hopefully this makes more sense!

Before

This also must be run before the TurboNavigator instance is created, otherwise the block will have already executed and the custom web view configuration will not apply.

TurboConfig.shared.makeCustomWebView = { configuration in
    configuration.applicationNameForUserAgent? +=
        " \(Strada.userAgentSubstring(for: BridgeComponent.all))"

    let webView = WKWebView(frame: .zero, configuration: configuration)
    Bridge.initialize(webView)
    return webView
}

After

extension SceneDelegate: TurboNavigationDelegate {
    func webView(configuration: WKWebViewConfiguration) -> WKWebView {
        configuration.applicationNameForUserAgent? +=
            " \(Strada.userAgentSubstring(for: BridgeComponent.all))"

        let webView = WKWebView(frame: .zero, configuration: configuration)
        Bridge.initialize(webView)
        return webView
    }
}

[joemasilotti]

Closed in favor of #74.