Introduce shimmering loader placeholders

[nivisi]

1. A widget that adds a preconfigured shimmering parent.

2. A widget for building the actual shapes, e.g.
   

    /// Pre-configured widget for adding loading shapes to the screen.
    class PlaceholderShape extends StatelessWidget {
      const PlaceholderShape({
        super.key,
        this.width,
        this.height,
        this.borderRadius,
        this.color,
      });
    
      const PlaceholderShape.square({
        super.key,
        double? size,
        this.borderRadius,
        this.color,
      })  : width = size,
            height = size;
    
      /// A rectangular shape that has the size of the text with the given style.
      PlaceholderShape.fromText({
        super.key,
        required String text,
        required TextStyle textStyle,
        this.borderRadius,
        this.color,
      })  : width = text.getSize(textStyle).width,
            height = text.getSize(textStyle).height;  
    
      final double? width;
      final double? height;
      final double? borderRadius;
      final Color? color;  
    
      @override
      Widget build(BuildContext context) {
        // TODO: Setup the default colour.
        final defaultColor = Colors.white54;
        
        return ClipRRect(
          borderRadius: BorderRadius.all(Radius.circular(borderRadius ?? .0)),
          child: SizedBox(
            width: width,
            height: height,
            child: ColoredBox(color: color ?? defaultColor),
          ),
        );
      }
    }

3. So it can be used like this:

   AppShimmer(
     child: Column(
       children: [
           LoadingPlaceholderShape(...),
           LoadingPlaceholderShape(...),
       ],
     ),
   // or
   AppShimmer.column(
     children: [
       LoadingPlaceholderShape(...),
       LoadingPlaceholderShape(...),
     ],
   ),
   // or
   AppShimmer.row(
     children: [
       LoadingPlaceholderShape(...),
       LoadingPlaceholderShape(...),
     ],
   ),