addVectors(a, b) {
    this.x = a.x + b.x;
    this.y = a.y + b.y;

    return this;
  }
Adds two vectors and sets the result to this vector.
@param {Vector2D} a - The first vector to add.
@param {Vector2D} b - The second vector to add.
@returns {Vector2D} This vector.