Scaffold nannou bevy refactor

[charlotte]

In order to get started on the bevy refactor, we'll need a minimal set of crates. Here's what I'm thinking to start.

• bevy-nannou - the "new" top level crate that will be eventually renamed nannou.
• bevy-nannou-wgpu - port of the nannou-wgpu crate to work with bevy's wgpu types and render apis.
• bevy-nannou-render - implementation of the existing render pipeline needed for the draw api.
• bevy-nannou-draw - the draw api. This is currently in the top level crate, but I think makes sense to be split into it's own crate representing a bevy plugin.

Right now, we're choosing to following the naming convention that places our plugins "inside" the bevy ecosystem. I think this makes sense since some of these could be used by projects outside of nannou, but it also would be totally fine to prefix as nannou-bevy-*. Something to consider!