add(v, w) {
    if (w !== undefined) {
      return this.addVectors(v, w);
    }

    this.x += v.x;
    this.y += v.y;

    return this;
  }
Adds another vector to this one.
@param {Vector2D} v - The vector to add.
@param {Vector2D} [w] - An optional second vector to add.
@returns {Vector2D} This vector.