rotate(tha) {
    const x = this.x;
    const y = this.y;

    this.x = x * Math.cos(tha) + y * Math.sin(tha);
    this.y = -x * Math.sin(tha) + y * Math.cos(tha);

    return this;
  }
Rotates this vector by an angle.
@param {number} tha - The angle to rotate by (in radians).
@returns {Vector2D} This vector.