Half-size Gridfinity using 21mm base size

For folks with small printers, or who need to store small parts, or who wish to fit baseplates into odd sizes.

Initial BlockSCAD implementation at:

https://www.blockscad3d.com/community/projects/1988709

Based on Zack Freedman's https://www.youtube.com/watch?v=ra_9zU-mnl8

Dimensions pulled from: https://gridfinity.xyz/specification/

Note that at least one generator has a configurable base grid size, so should handle this already: https://gridfinity.perplexinglabs.com/pr/gridfinity-rebuilt/0/0