* chore: general maintenance

* chore: removed the unused `async` dependency
* chore: added a version constraint to the `lint` dev dependency
* chore: resolved all analyzer lints (`unnecessary_library_name`,
  `simplify_variable_pattern`, `async_return_with_no_await`, `unnecessary_async`)
* ci: updated the GitHub actions to their latest major versions and replaced the
  flutter setup in the test job with the dart setup
* ci: fixed the unquoted variable expansions in `bin/ensure_pana_score.sh`
* doc: fixed the outdated/broken references in the readme and the dartdoc
  comments (the return type is a `FutureOr`, `Failed` exposes `reason`)
* chore: aligned `.editorconfig` with what `dart format` produces and added
  a `.gitattributes` to normalise the line endings

* fixes

* docs: fix the case-mismatched `Return value` anchors in the readme

GitHub slugifies headings to lowercase, so `## Return value` generates the
fragment `#return-value`. Both links used `#Return-value` and therefore did
not navigate to the section.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017b5dUmFrwLWxd8a85FN6Uy

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>