f32 and f64 additive features

[johanhelsing]

Another option. I wanted to see what it would take to have both f32 and f64 function at the same time.

This makes each implementation available under bevy_xpbd_3d::single::XpbdPlugin and bevy_xpbd_3d::double::XpbdPlugin. If only one of them is defined, adds re-exports at the crate level, i.e. bevy_xpbd_3d::XpbdPlugin.

Not sure what I think of it though, but just thought I'd share.

[Jondolf]

Interesting idea, haven't seen it before. This does maybe feel a bit hacky though, and seems to basically accomplish the same thing as making separate crates for f32 and f64, except with features and modules

[johanhelsing]

Yeah, definitely feels hacky.

Another option would be to use generics, so XpbdPlugin<f32> for instance, and then use associated types for everything (XpbdPlugin<f32>::Vector would be Vec3)... if it's even doable it would probably be a lot less readable, and everything would probably have to go into impl blocks.