Use provider as state management tool for list of tasks

ref: https://github.com/dwyl/flutter-todo-list-tutorial/issues/21#issuecomment-742535367

Provider can be use to make it easier to manage the state of the application, in particular to listen to changes applied to the list of tasks (e.g add a new task to the list, toggle complete/uncomplete task from the list...).
So instead of letting the widget managing their own state we can use provide to create one source for the state application which will define how to display the tasks.

see also https://flutter.dev/docs/development/data-and-backend/state-mgmt/simple