feat: support icon finders in tests

[adil192]

Closes #198

The find.byIcon and find.widgetByIcon use this finder internally:

class _IconWidgetFinder extends MatchFinder {
  _IconWidgetFinder(this.icon, {super.skipOffstage});

  final IconData icon;

  @override
  String get description => 'icon "$icon"';

  @override
  bool matches(Element candidate) {
    final Widget widget = candidate.widget;
    return widget is Icon && widget.icon == icon;
  }
}

This PR makes FaIcon extend Icon, reducing unnecessary code and allowing those finders to work.