How to launch/load local html for iOS?

I have a html file in assets/index.html, I can launch it for Android use the url like:

```dart
String localUrl = 'file:///android_asset/flutter_assets/assets/index.html';
```

What url should I use for iOS? Following is the launch code:

```dart
_flutterWebviewPlugin.launch(
localUrl,
ignoreSSLErrors: true,
withLocalUrl: true,
localUrlScope: 'assets/',
allowFileURLs: true,
hidden: true
);
```

该 issue 指出了 assets/index.html 和 _flutterWebviewPlugin.launch 调用，但没有实现文件或测试。首先检查 plugin 的 iOS 本地文件处理方式以及现有的使用示例。完成标准是形成一个有文档记录且经过验证的 iOS 加载模式。

由索引模型根据 Issue 内容生成。