[extensions] Signal/Computed with $

[rodydavis]

Been thinking about ways to simplify the extension methods for creating signals for core types.

Given the following extensions:

extension SignalObjectUtils<T extends Object> on T {
  Signal<T> get $ => signal<T>(this);
}

extension SignalOptionalObjectUtils<T extends Object> on T? {
  Signal<T?> get $ => signal<T?>(this);
}

extension ComputedObjectSignalUtils<T extends Object> on T Function() {
  Computed<T> get $ => computed(this);
}

extension ComputedOptionalObjectSignalUtils<T extends Object> on T? Function() {
  Computed<T?> get $ => computed(this);
}

We could then define signals as follows:

void main() {
  final a = 1.$;
  final b = 2.$;
  final c = (() => a() + b()).$;

  effect(() {
    print(c.value); // 3
  });
}

This also disambiguates the extensions for converting types to signals ValueNotifier/ValueListenable/Stream/Future since they conflict with Object.