Snapmaker 2 Extra Part Cooling Fan

(Hopefully) final revision of the 5015 part cooling fan mount idea from [brent113's forum post](https://forum.snapmaker.com/t/3dp-part-cooling-fan-5015-replacement/10038?u=nivekmai).

Designed with a few goals in mind:

- extremely easy to print in PETG (you don't want to print the fan mount in PLA if you ever want to print hotter materials), absolutely no supports needed
- easily removable for servicing the hotend/toolhead/fan mount itself
- most direct air flow path possible, to avoid turbulent air causing print issues

NOTE: It's become apparent that the precision of your slicer in the vent is important, there are lots of very thin walls in it, and you need to check your slicer's output when printing them to make sure that it's actually printing all the walls. See [my post in the forum](https://forum.snapmaker.com/t/3dp-part-cooling-fan-5015-replacement/10038/267?u=nivekmai) where I was diagnosing this, make sure your slicer is spitting out the correct gcode before printing,