Primitive mesh construction parameters

[RoboMage]

Small and somewhat breaking change idea : Bevy primitives should be using half-extents instead of full extents.

example :

Capsule takes in a radius and a full length

Capsule3d::new(radius : 0.5,  length : 1.0)

...but immediately converts to half_length

pub fn new(radius: f32, length: f32) -> Self {
        Self {
            radius,
            half_length: length / 2.0,
        }
    }

I feel like it doesn't make any sense to have this mix and match. If you're using full lengths, then you'd expect a diameter instead of a radius. However, most game engines and physics systems deal with half extents and radii. Bevy should switch to this convention.

I wanted to bring this up on the discussion board before doing a pull request because I'm not sure how many things this would break.