Add `isWebOrDesktop` to ResponsiveBuilder

[georgeherby]

Fixes #31

Fixes the issues on all Desktop platforms when the window is wider than the height and mean it used the tablet sizes.

This change ensures that width=Size.width rather than width=Size.shortest when on Web and Desktop. The code was there in the getDeviceTypes and getRefindedSize but it was either was not configurable and stuck as kIsWeb or had a parameter and defaulted. I have added a parameter to ResponsiveBuilder to allow this to be overridden. Currently, it just defaults to kIsWeb as this was the existing behaviour.

Example

ResponsiveBuilder(
      isWebOrDesktop: kIsWeb || Theme.of(context).platform.isDesktop(), // Using an extension function to determine isDesktop
      builder: (context, sizingInformation) {
 ...
}

Extra. If anyone wants the extension function to use the above

extension PlatformCheck on TargetPlatform {
  bool isDesktop() {
    return [TargetPlatform.macOS, TargetPlatform.windows, TargetPlatform.linux]
        .contains(this);
  }
}

To use the code in this PR in your app:

  responsive_builder:
    git:
      url: git://github.com/georgeherby/responsive_builder.git
      ref: master