Annotate extension methods with @widgetFactory #64

Flutter has recently introduced the @widgetFactory 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:

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.