widget/update_sliver library

1. desktop_updater package
2. documentation
3. widget/update_sliver.dart

desktop_updater package

1. Libraries
2. desktop_updater
3. desktop_updater_inherited_widget
4. desktop_updater_method_channel
5. desktop_updater_platform_interface
6. updater_controller
7. widget/release_notes_bottom_sheet
8. widget/update_card
9. widget/update_dialog
10. widget/update_direct_card
11. widget/update_problem_report_dialog
12. widget/update_sliver
13. widget/update_widget