Add support for refined layout sizes

[SiyrisSoul]

Added RefinedLayoutBuilder which supports small, normal, large and extraLarge layouts for each device type. Also added getValueForRefinedSize. Haven't updated example, readme or changelog.

You can use RefinedLayoutBuilder like this:

ScreenTypeLayout.builder(
  mobile: (context) => RefinedLayoutBuilder(
     small:  (context) => HomeMobileSmall(),
     normal: (context) => HomeMobileNormal(),
     large: (context) => HomeMobileLarge(),
     extraLarge: (context) => HomeMobileExtraLarge()
  ),
);

You can use getValueForRefinedSize like this:

getValueForScreenType<double>(
  context: context,  
  mobile: getValueForRefinedSize<double>( 
     context: context, 
     small: 10,
     normal: 15,
     large: 30,
  ),
);

You can set refinedBreakpoints globally like this:
If any breakpoints aren't set then defaults will be used. This makes it easy to just edit one platform without worrying about 12 different breakpoints.

void main() {
  ResponsiveSizingConfig.instance.setCustomBreakpoints(
    ScreenBreakpoints(desktop: 800, tablet: 550, watch: 200),
    customRefinedBreakpoints: RefinedBreakpoints(desktopSmall: 950, desktopNormal: 1920, desktopLarge: 3840, desktopExtraLarge: 4096),
  );
  runApp(MyApp());
}