Updated package to Flutter 3 and Dart 2.17 and cleaned up a little bit

[novas1r1]

Hey, I just worked with the plugin and then somehow upgraded it and cleaned up some things to match some rules for flutter packages based on the official documentation.

Please let me know what you think about it :)!

• Updated example to Flutter 3 and Dart 2.17
• BREAKING: only package:intro_slider/flutter_intro_slider.dart has to be imported now to get access to the whole feature
• BREAKING: "dotSliderAnimation" is now "DotSliderAnimation" to match enum guidelines
• BREAKING: "scrollbarBehavior" is now "ScrollbarBehavior" to match enum guidelines
• Internal: Project structure cleanup
• Internal: Example cleanup
• Internal: Added flutter_lints
• Internal: Renamed files to match the class name
• Internal: Removed home.dart from example

[duytq94]

Hi, It's very nice, thanks a lot 👍