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.

[michaelspiss]

This is awesome on so many levels and something we probably should have done earlier. Thank you so much @adil192!