Add Oref-like composition API package

[medz]

I'm currently having issues with how I handle the effect flags, so it won't trigger the Widget rebuild correctly.

CC @rodydavis

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

  @override
  Widget build(BuildContext context) {
    final count = signal(context, 0);

    return Scaffold(
      appBar: AppBar(title: const Text('Counter')),
      body: Center(child: Text('Count: ${count.value}')),
      floatingActionButton: FloatingActionButton(
        onPressed: () => count.value++,
        tooltip: 'Increment',
        child: const Icon(Icons.plus_one),
      ),
    );
  }
}

[medz]

The evalContext in preact_signals isn't collected correctly as I'd expected simply by setting it.

Instead, it doesn't have direct collection capabilities. Ideally, the flow is:

1. .value -> track effect (set evalContext)
2. value changed -> trigger effect
3. rebuild -> goto at 1