Add glam swizzles traits to prelude

[tguichaoua]

Objective

Add possibility to use the glam's swizzles traits without having to manually import them.

use bevy::prelude::*;
- use bevy::math::Vec3Swizzles;

fn foo(x: Vec3) {
    let y: Vec2 = x.xy();
}

Solution

Add the swizzles traits to bevy's prelude.

---

Changelog

• Vec2Swizzles, Vec3Swizzles and Vec4Swizzles are now part of the prelude.

[alice-i-cecile]

This sort of utility trait is a good fit for the prelude IMO.

[killercup]

None of the methods are dangerous to overlap with some other trait, and to me it's nice to have more features discoverable by default :)