static angle(vecA, vecB) {
    let dot = Vector2.dot(vecA, vecB);
    return Math.acos(dot / (vecA.length() * vecB.length()));
  }
Returns the angle between two vectors.

@static
@param {Vector2} vecA A vector.
@param {Vector2} vecB A vector.
@returns {Number} The angle between two vectors in radians.