Add copysign operation and unit tests for it

[atlv24]

Just mirror's Rust's existing f32/f64::copysign functionality but vectorized component-wise.

It turns out that this operation is exactly whats needed when computing the support point of a box given its extents and a direction. Example usage from a physics engine I'm working on that prompted me to make this PR:

fn support_point(&self, dir: Vec3A) -> Vec3A {
    self.extents.copysign(dir)
}