Rename Direction2d/3d to Dir2/3

[Jondolf]

Objective

Split up from #12017, rename Bevy's direction types.

Currently, Bevy has the Direction2d, Direction3d, and Direction3dA types, which provide a type-level guarantee that their contained vectors remain normalized. They can be very useful for a lot of APIs for safety, explicitness, and in some cases performance, as they can sometimes avoid unnecessary normalizations.

However, many consider them to be inconvenient to use, and opt for standard vector types like Vec3 because of this. One reason is that the direction type names are a bit long and can be annoying to write (of course you can use autocomplete, but just typing Vec3 is still nicer), and in some intances, the extra characters can make formatting worse. The naming is also inconsistent with Glam's shorter type names, and results in names like Direction3dA, which (in my opinion) are difficult to read and even a bit ugly.

This PR proposes renaming the types to Dir2, Dir3, and Dir3A. These names are nice and easy to write, consistent with Glam, and work well for variants like the SIMD aligned Dir3A. As a bonus, it can also result in nicer formatting in a lot of cases, which can be seen from the diff of this PR.

Some examples of what it looks like: (copied from #12017)

// Before
let ray_cast = RayCast2d::new(Vec2::ZERO, Direction2d::X, 5.0);

// After
let ray_cast = RayCast2d::new(Vec2::ZERO, Dir2::X, 5.0);

// Before (an example using Bevy XPBD)
let hit = spatial_query.cast_ray(
    Vec3::ZERO,
    Direction3d::X,
    f32::MAX,
    true,
    SpatialQueryFilter::default(),
);

// After
let hit = spatial_query.cast_ray(
    Vec3::ZERO,
    Dir3::X,
    f32::MAX,
    true,
    SpatialQueryFilter::default(),
);

// Before
self.circle(
    Vec3::new(0.0, -2.0, 0.0),
    Direction3d::Y,
    5.0,
    Color::TURQUOISE,
);

// After (formatting is collapsed in this case)
self.circle(Vec3::new(0.0, -2.0, 0.0), Dir3::Y, 5.0, Color::TURQUOISE);

Solution

Rename Direction2d, Direction3d, and Direction3dA to Dir2, Dir3, and Dir3A.

---

Migration Guide

The Direction2d and Direction3d types have been renamed to Dir2 and Dir3.

Additional Context

This has been brought up on the Discord a few times, and we had a small poll on this. Dir2/Dir3/Dir3A was quite unanimously chosen as the best option, but of course it was a very small poll and inconclusive, so other opinions are certainly welcome too.