Add shadows to theme

[jorre127]

Add shadows to theme so they are easily reusable and all the shadows are the same in the app. Something like:

class FlutterTemplateShadows {
  late final List<BoxShadow> cardShadow;

  FlutterTemplateShadows({required Color shadow}) {
    cardShadow = [
      BoxShadow(
        color: shadow,
        offset: const Offset(1, 1),
        blurRadius: 2,
      ),
      BoxShadow(
        color: shadow,
        offset: const Offset(3, 3),
        blurRadius: 4,
      ),
    ];
  }
}

And then in your theme:

FlutterTemplateTheme._(
         ...
        shadows: SPShadows(shadow: colorTheme.shadow),
      );