[webview_flutter] Implement onJSAlert, onJSConfirm, onJSPrompt interface

[j796160836]

Description

Enhance pull request of #2012 .

Implement onJSAlert, onJSConfirm, onJSPrompt interface.

Instead of implement native code, I advocate it handling that Flutter side.
Let developer implement their own alert dialogs at Flutter.
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	JSAlertCallback onJSAlert;
Confirm	JSConfirmCallback onJSConfirm;
Prompt	JSPromptCallback onJSPrompt;

Also, add working examples.

Related Issues

• Issue webview_flutter plugin JavaScript popup boxes are not shown flutter#34853
• Issue alert() method doesn't work in WebView flutter#30358
• Pull request [webview_flutter] Add support for js alert, confirm, prompt. #2012

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[j796160836]

@googlebot I fixed it.

[j796160836]

I had signed CLA at 2015 but seems googlebot can't fetch my CLA.

[cyanglaz]

I'm going to close the PR because the CLA is not signed. Feel free to ping me when the CLA is signed and I'll re-open the PR. Or you can open a new PR after signing the CLA.