add sizing extension members to num class

lib/size_extension.dart

+import 'flutter_screenutil.dart';
+
+extension SizeExtension on num {
+  num get w => ScreenUtil().setWidth(this);
+
+  num get h => ScreenUtil().setHeight(this);
+
+  num get sp => ScreenUtil().setSp(this);
+
+  num get ssp => ScreenUtil().setSp(this, allowFontScalingSelf: true);
+}