Add Dart 3 and current Flutter compatibility

[RockerFlower]

Context

The package still declares sdk: >=2.12.0 <3.0.0. The first maintenance PR intentionally keeps that constraint unchanged to avoid a breaking compatibility jump.

Goal

Evaluate and add support for Dart 3 and current stable Flutter without breaking existing users on the supported Dart 2 line unless a release plan explicitly accepts that change.

Suggested scope

• Decide the minimum Dart and Flutter versions for the next release.
• Run the package and example against a current stable Flutter SDK.
• Update pubspec.yaml SDK constraints only after compatibility is verified.
• Refresh lockfiles with the chosen SDK.
• Add CI coverage for the current stable Flutter version, possibly as a matrix alongside Flutter 3.7.
• Document any required migration notes.

Acceptance criteria

• flutter analyze passes on the selected SDK matrix.
• flutter test passes on the selected SDK matrix.
• flutter pub publish --dry-run passes.
• README or CHANGELOG explains the supported SDK range.

[hiSandog]

Would the next release continue supporting Dart 2.12, or is raising the lower bound acceptable if current Flutter dependencies require it? A useful CI matrix would pair the oldest retained SDK with current stable Flutter, since testing only the newest SDK can miss accidental use of newer language features. The example app should be included because plugin API breakage may not appear in package-level analysis.