ExtrusionThickness added to SOLID entity.

[a-alexander]

Hi there,

I needed the extrusion thickness on the solid entity (since without it you've got no extrusion depth dimension). Autodesk spec lists under the 39 flag on a solid entity, so I've added to suit.

I'm not usually a js/ts dev so bit out of my depth here. Hopefully it's all ok. Sorry, no tests.

Thanks,

Adam