Add demo using bevy

[ayrtonm]

Bevy is adding no_std support so it will be possible to use a subset of it on the PS1. Once it supports platforms that lack CAS and other atomic operations it'd be nice to add a demo using bevy to this repo.

[BenjaminBrienen]

This should be possible to play with now using Bevy/main.

[ayrtonm]

nice, thanks for the heads up. I don't have time to test it out for a good while but if anyone is using it and puts together some demo PRs are welcome.