Fixed AsyncTransition

[audioXD]

Describe the bug
The current implementation AsyncTransition::run is not sound.

Leptos Dependencies

reactive_graph = "0.2.0-rc2"

To Reproduce

#[tokio::test]
async fn test_logic() {
    use std::time::Duration;
    use or_poisoned::OrPoisoned as _;
    use reactive_graph::transition::AsyncTransition;

    tokio::join!(
        AsyncTransition::run(move || async move {
            tokio::time::sleep(Duration::from_millis(1)).await;
            tokio::time::sleep(Duration::from_millis(1)).await;
        }),
        AsyncTransition::run(move || async move {
            tokio::time::sleep(Duration::from_millis(2)).await;
            tokio::time::sleep(Duration::from_millis(2)).await;
        }),
    );

    // Made `global_transition()`and `TransitionInner` `#[doc(hidden)]` + `pub` for this to work
    assert!(reactive_graph::transition::global_transition().write().or_poisoned().is_none(), "A MPSC is still registered")
}

Next Steps
[x ] I will make a PR
[ x] I would like to make a PR, but need help getting started
[ x] I want someone else to take the time to fix this
[x ] This is a low priority for me and is just shared for your information

[audioXD]

I just realized that we could just use a plain Vec instead of mpsc now...

[audioXD]

And I now see that there's an issue where it doesn't pierce into a nested spawn(async move {}); The old implementation kinda did that by accident (at least if the spawned task finished before the main future resolved)