feat: add YouTube RSS feed subscription (#38)

- Add rss_dart dependency
- Add Feed page
- Move configPathLookup to const.dart
- Video duration is now optional on ListItemVideo
- Error thumbnails now show an icon
- Add setting entry for feed list
- Add helper functions to read and write to feed.yaml
- !Feature regression: video history disabled due to support for videos from Feed, will be added back in #29

ci: fix build.yml step order

feat: migrate from SharedPreferences to CSV based configuration file (#…

…36)

ci: add Github Action workflows

- Specified Flutter environment version in pubspec.yaml
- Add version constraint to `custom_lint` and `riverpod_lint`
- Add check.yml workflow
- Add build.yml workflow