Outer cherry bevel

[AaronVerDow]

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I was having trouble fitting some key caps into Kailh box cherrys due to the outer edge. This pull request adds an optional outer bevel for the cherry stems (box, rounded, and normal.)

There were some hard coded numbers in the stems scad files that I couldn't identify at first so I made them constants or settings.

[jdart]

What stem type did you find worked best for box switches?