feat: Signal interface

[mfulton26]

improves inferred types and developer experience for signals

before:

completed: {
    (): boolean;
    (value: boolean): void;
}

after:

completed: Signal<boolean>

[bolt-new-by-stackblitz]

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[mfulton26]

❓ Would it be feasible to have interfaces for Computed, Effect, and EffectScope too? If so, it could be valuable to have is{Signal,Computed,Effect,EffectScope}() be typed as type assertion functions (e.g. something like function isSignal<T = void>(fn: () => T): fn is Signal<T>).