Generate planets

[thombruce]

Given a seed, generate a planet. Use a planetary generator to populate a solar system rather than reusing art assets and/or filling the repository with such assets manually.

For the time being, provide orbital radii manually for a single system.

[thombruce]

Long term, I would like to port https://github.com/Deep-Fold/PixelPlanets to Rust so that we can use the excellent animated planets generated there.

But that's not a small or clear task.

In the meantime, maybe make use of https://github.com/khonsulabs/magrathea/ which is already written in Rust and should work no problem in Bevy.