Add perp method to Dir2

[ParvePalial]

Fixes #25418

This PR adds a perp method to Dir2 inline with how glam implements it for Vec2. It utilizesSelf::new_unchecked since rotating a normalized 2D unit vector by 90 degrees inherently preserves its unit length.

Unit tests have been added to verify correct rotation and length preservation.

[github-actions]

Welcome, new contributor!

Please make sure you've read our contributing guide, as well as our policy regarding AI usage, and we look forward to reviewing your pull request shortly ✨

[JMS55]

Any reason not to call it perpendicular ?

[Jengamon]

I agree with @JMS55 about naming the method perpendicular, with 2 main points:

• this keeps code like the one changed in bevy_gizmos working without change
• it follows the general pattern I notice elsewhere in bevy_math where operations tend to be spelled-out

[ParvePalial]

I agree with perpendicular() method if complete names are common practice
and thanks for correcting out the - typo in counterclockwise

[hukasu]

The one vantage of calling it perp is that it would shadow the impl accessible through the deref

[ParvePalial]

I am happy to change it back to perp if the core maintainers prefer that behavior, but I will leave it as perpendicular() for now based on the previous feedback.