Fix: restrict setHeading() to 2D vectors and add friendly error for others

[reshma045]

Resolves #8215

Changes:

This PR updates the p5.Vector.setHeading() method to support only 2D vectors in alignment with the proposed 2.x design.

• Added a dimension check in setHeading():
• If the vector’s z component is nonzero (i.e., not 2D), the method now emits a clear p5._friendlyError() explaining that setHeading() is 2D-only in p5.js 2.x.
• For 2D vectors (z === 0), heading behavior remains the same, preserving magnitude and rotating in the xy-plane.
• Keeps compatibility with existing sketches that use setHeading() on true 2D vectors.

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated