Update winit_runner to use spawn_app for wasm32 target

[xuwaters]

Objective

• Fixes Allow winit event loop to be recreated on web / wasm #12562
• Fixes Need a proper way to handle mounting and remounting Bevy in a SPA (React/Angular/Vue) #12195

Solution

• Use spawn_app instead of run_app for web platform in winit_runner as suggested in the document

Testing

• Did you test these changes? If so, how?

Tested on web. Created a react app which renders the bevy WASM app and returns the disposer to JS. Js will call the disposer on component unmount to stop the app, the disposer sends a signal to a signal receiver in rust which exits the app like this:

fn handle_stop_signal(
    signal: NonSendMut<StopSignalReceiver>,
    window_entities: Query<(Entity, &Window)>,
    mut event_writer: EventWriter<WindowCloseRequested>,
) {
    if let Ok(_) = signal.try_recv() {
        for (entity, _window) in window_entities.iter() {
            info!("closing window entity: {:x}", entity.to_bits());
            event_writer.send(WindowCloseRequested { window: entity });
        }
    }
}

• Are there any parts that need more testing?

  • No

• How can other people (reviewers) test your changes? Is there anything specific they need to know?

  • Are all resources released after stopping the app like this? The WASM is still loaded, the LogPlugin complains on the logger re-initialization, but it's a warning anyway.

• If relevant, what platforms did you test these changes on, and are there any important ones you can't test?

  • Tested the WASM version on web platform and the native version on MacOS.

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[BenjaminBrienen]

Maybe "this is a bug" could be improved to recommend whether to open an issue in bevy or winit (whichever is relevant, idk)

[tysen]

I just ran into the problem this PR solves and was going to make this change myself - was happy to see it already awaiting merge!