Fix setHeading behavior for non-2D p5.Vector instances

[aashu2006]

Resolves #8215

Changes:

• Restrict p5.Vector.prototype.setHeading() to vectors with a zero or undefined z component.
• Throw an error when setHeading() is called on vectors with a non-zero z value.
• Added unit tests to cover invalid-dimension behavior.

Screenshots of the change:
N/A (non-visual change)

PR Checklist

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

[aashu2006]

Happy to adjust this if a different error behavior or messaging is preferred.