Ender 3 Dual-Drive Extruder Mod

This mod adds dual-extrusion to an ender 3 in the simplest way possible by directly replacing the extruder idler arm. The only parts necessary for this upgrade, besides the original hardware and 3d printed parts, are a set of Bondtech-style extruder gears. The gear spacer is optional, but it makes aligning the gear on the motor shaft much easier. If the previous idler arm has a metal insert to feed filament into the extruder, remove it and press it into the new idler arm with a soldering iron. If not, glue a piece of Bowden tube into the hole