K8200/3DRAG extruder core with flex support and slot mount base

Completely redesigned in OpenSCAD to lower the base of the hotend and make it easier to fit a slot mount hotend in place of the original

- Updated to easily switch between 3 and 1.75mm paths, and to replace steep overhangs with small bridges on recessed nut sockets.

- Further updated to widen the whole thing slightly, to simplify the geometry a little and to remove thin walls that are hard to print