Add extension on double

Replace extension on num with that on double.

Author: sidrao2006

lib/size_extension.dart

@@ -1,6 +1,6 @@
 import 'package:flutter_screenutil/screenutil.dart';
 
-extension SizeExtension on num {
+extension SizeExtension on double {
   ///[ScreenUtil.setWidth]
   num get w => ScreenUtil().setWidth(this);