static units(vecA, vecB) {
    let delta = Vector2.subtract(vecB, vecA);
    return [new Vector2(-delta.y, delta.x).normalize(), new Vector2(delta.y, -delta.x).normalize()];
  }
Returns the unit (normalized normal) vectors of a line spanned by two vectors.

@static
@param {Vector2} vecA A vector spanning the line.
@param {Vector2} vecB A vector spanning the line.
@returns {Vector2[]} An array containing the two unit vectors.