Rewrite checkbox(task list) extension (dart-lang/markdown#450)

* revert to old version

* Parse tasklist items

* Add test cases

* Change package import to path import

* Add a comment to UnorderedListSyntax

* Some optimisation

1. Remove enableCheckbox option from ListSyntax
2. Rename UnorderedListWithCheckBoxSyntax to UnOrderedListWithCheckboxSyntax
3. Rename OrderedListWithCheckBoxSyntax to OrderedListWithCheckboxSyntax

* Fix a prefer_interpolation_to_compose_strings issue

* Fix the requested changes

* Put an accidentally deleted test case back

* Rename successfulMatch to match

* Rename UnOrderedListWithCheckboxSyntax to UnorderedListWithCheckboxSyntax