feat: add flutter-version datasource (#14743)

* feat: add flutter datasource

* refactor: rename flutter datasource to flutter-version

* feat: update to handle duplicate flutter releases

* refactor: tidy up flutter version datasource class names and imports

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

* Apply suggestions from code review

* style: add newline to test file

* refactor: move regex to util module

* refactor: tidy up stable version regex

* Apply suggestions from code review

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>
Co-authored-by: Rhys Arkins <rhys@arkins.net>