Add Trigger::dispose

[luxalpa]

Is your feature request related to a problem? Please describe.
I currently use RwSignal<()> as a trigger because it allows me to dispose it manually. I don't see a reason why we wouldn't want to have that same functionality on Triggers.

Describe the solution you'd like

impl<T> SignalDispose for Trigger {
    fn dispose(self) {
        _ = with_runtime(|runtime| runtime.dispose_node(self.id));
    }
}

Describe alternatives you've considered
Currently I'm using RwSignal<()>. To be fair I am not sure if there's any actual advantages of using Trigger over RwSignal<()>

Additional context
I'm using this to implement my global state in some kind of subscriber-system where signals are being created ad-hoc only for the elements that are visible on the page instead of for the entirety of the global state (which can have a large number of objects).