fix: exiting the event loop

[ThePoultryMan]

Objective

Currently, if the bevy app exits, the program will hang because the event loop is never exited.

Solution

Fixed by checking if the bevy app has an exit signal, and breaking the event loop if it does.

Testing

• Tested on my own app.
• Tested on both examples.

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Migration Guide

Technically there are no breaking changes, however, for consistency with using bevy apps, instead of using std::process::exit, users should use the EventWriter<AppExit> instead.

Both examples have been updated to account for this.