Updated codebase for Dart minimum 2.12.0

[ZanderCowboy]

• Added flutter_lints to all samples.
• Fixed linting error for all the samples
• Fixes for contacts, updated.
• Added web version for contacts
• Added a top level pubspec
• Fixes for todo, updated.
• Fixes for sketchpad, updated.
• Fixes for favorite_books, updated.
• Added web version for favorite_books
• Implemented fixes for favorite_books main
• Fixes for counter, updated.
• Implemented fixes for counter main
• Fixes for backup_restore_books, updated.
• Added web version for backup_restore_books
• Implemented fixes for backup_restore_books main
• Create README.md

This PR is for an updated codebase that works with Dart minimum SDK version 2.12.0. Flutter linting were also added and any changes were added/implemented.