Annotate extension methods with `@widgetFactory`

Flutter has recently introduced the [`@widgetFactory`](https://master-api.flutter.dev/flutter/widgets/widgetFactory-constant.html) annotation to improve the dev UX for widgets created in extension methods. It's currently only available in the master channel, so implementing this will have to wait until the next stable feature release and a point at which this package is fine with requiring that version of Flutter.

Take this example:

```dart
import 'package:flutter/material.dart';
import 'package:flutter_animate/flutter_animate.dart';

void main() {
runApp(const SizedBox().animate());
}
```

Currently, the `Animate` widget, created in `animate`, is not included in the summary widget tree in the widget inspector:


After annotating the `animate` method with `@widgetFactory` the `Animate` widget is included in the summary widget tree like if `Animate` was used directly and not through the extension method.