feat: Implement Default trait for RwSignal and StoredValue

[luoxiaozero]

Implement Default trait for RwSignal and StoredValue so that optional properties can be implemented, such as:

#[component]
fn My(#[prop(optional)] value: RwSignal<f32>) -> impl IntoView {
    ...
}


fn main() {
   view! {
       <My />
   }
}

[gbj]

Thanks! I could've sworn I did this already but maybe it was in a dream 😅