Improved Ender 3 Stealthburner mount (MGN12 linear rail)

This model is a slight remix of Kejar31's Stealthburner mount for the Ender 3.

Changes include:

• Added toolhead “foot” to make alignment of the toolhead easier and more rigid
• Added threaded insert holes instead of self-tap holes to hold the top of the toolhead
• Added support-free geometry to the floating holes in the left half
• Redesigned a much more rigid BLTouch mount from scratch

Assembly notes:

• One of the heat set inserts on the BLTouch mount is close to the vertical mount, be careful not to melt it.  Applying a piece of kapton tape before inserting can help.
• The heat set inserts on the right half of the block that are used for connecting the BLTouch mount often bulge out the front of the model.  These need to be filed flush, even if it causes the insert to break through the front wall, or else the toolhead will not be able to properly mount flush to the block.

Probe Offsets:

X=36
Y=12
Z=-2*

* Z offset will vary depending on which toolhead and nozzle are installed