Add `get_point` helper to `RayCaster`

[Shatur]

Right now to get position users need to take origin, add direction and multiply by distance:

fn print_hits(query: Query<(&RayCaster, &RayHits)>) {
    for (ray, hits) in &query {
        for hit in hits.iter() {
            println!(
                "Hit entity {} at {}",
                hit.entity,
                ray.origin + *ray.direction * hit.distance,
            );
        }
    }
}

But it would be nice to have a helper like Ray3d::get_point.