Implement onJSAlert, onJSConfirm, onJSPrompt interface

[j796160836]

Related issue: #376, enhanced Pull request by: #522.

Instead of implement native code, I advocate it handling that flutter side. I extract both iOS and Android native callback into three functions.

Native callback

	Android	iOS
Alert	public boolean onJsAlert(WebView view, String url, String message, final JsResult result)	webView(_:runJavaScriptAlertPanelWithMessage:initiatedByFrame:completionHandler:)
Confirm	public boolean onJsConfirm(WebView view, String url, String message, final JsResult result)	webView(_:runJavaScriptConfirmPanelWithMessage:initiatedByFrame:completionHandler:)
Prompt	public boolean onJsPrompt(WebView view, String url, String message, String defaultValue, final JsPromptResult result)	webView(_:runJavaScriptTextInputPanelWithPrompt:defaultText:initiatedByFrame:completionHandler:)

Flutter callback

	Flutter
Alert	flutterWebViewPlugin.onJSAlert = (alert) async { };
Confirm	flutterWebViewPlugin.onJSConfirm = (confirm) async { };
Prompt	flutterWebViewPlugin.onJSPrompt = (prompt) async { };

Also, I filled up the example.

[charafau]

Thank you for PR and sorry for long review time, I am quite busy these days. I will review this asap, although code doesn't work for me for example project.

[shinriyo]

@j796160836 conflicted

[jfacoustic]

Any updates on this feature?