normalize() {
        // normalize
        let len = this._x * this._x + this._y * this._y;
        if (len > 0) {
            len = 1 / Math.sqrt(len);
        }
        this._x *= len;
        this._y *= len;

        return this;
    }
Normalize this vector

     returns:
     @this (Vec2): normalized vector
*