Add oref package

[medz]

Oref is very cool.

import 'package:oref/oref.dart';

class Counter extends StatelessWidget {
  const Counter({super.key});

  @override
  Widget build(BuildContext context) {
    final count = signal(context, 0);
    void increment() => count(count() + 1);

    return Column(children: [
      Text('Count: ${count()}'),
      TextButton(
        onPressed: increment,
        child: Text('click me'),
      )
    ]);
  }
}

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• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

[johnpryan]

Thank you for the PR, could you file an issue first? We generally look for pub.dev metrics that indicate package popularity before we add it as a default package in DartPad. Thank you!