upgrade lowcost mk3 to mk4 (add square nut captive gap)

Add a square nut captive gap for lower-inner linear bearing housing screws, Added pockets to top Z brackets to accommodate Z lead screws.

The captive gap is required since the geometrical changes to make this parts compatible with MK3S+ style extruder; ended up moving the lower smoot rod to partially cover the lower-inner housing screws. With this mod now a square nut and a M3x18mm screw can be used instead.

Follow the original printing instructions, no supports print smooth rod orifices facing down on the plate.

Notes (2023-04-26) requirements I found from original: 

• Bearings needed are LM10LUU (the long version)
• Bearing housing 
  • 4x M3x30mm for all the ones that go through
  • 4x M3x18 and 4x square nuts
• The idler needs a Pan type screw M3x20mm, the regular screw will hit/rub the Z lead screw
• Also found the hard way that this requires a longer X belt

v1.3  2023-04-29

• Added modify y-motor-holder, shorter top part to:
  • Allow MK3S+ Y frame to pass over the motor bracket
  • Pass belt-test axis lenght test

v1.2  2023-04-27

• Updated to Z lead screw top left and right, for a better gap with lead screw

v1.1  2023-04-26

• Fix issue with square nut hole in motor side bracket on the holes for Z lead screw nut This issue was present on original
• Fixed issue with top brackets where original Z lead screws are to long, added pocket to compensate, This came from the original Prusa MK4 Printable Parts 

v1.0  2023-04-24

• Initial Remix Release