Length of DistanceJoint cannot be set through DistanceJointDef

[spinthil]

Hallo,
I'm using a DistanceJoint to attract a DynamicBody to a static center Body. The original Box2D implementation allows setting the length of the DistanceJoint through DistanceJointDef. In planck-js the length is automatically calculated as the distance between the two anchors:

this.m_length = Vec2.distance(anchorB, anchorA);

Currently I'm overwriting m_length after creating the joint. It would be nice if the length could be specified in the definition.

[shakiba]

Thanks for reporting the issue! It would be great if you make a PR to fix it! :)