Design for resin printing differs in important ways from design for
FDM printing.

This change aims to improve printability when the base of the keys is
against the build plate of a resin printer.

The two printability concerns addressed are:
 * Fully enclosed volumes, called "suction cups". These volumes are
   undesirable, because the action of pulling up the build plate after
   exposing a layer creates a vaccum here which can make the print fail.

   By adding breaks at each of the 4 corners of the key, resin can freely
   flow into the central area and there are no suction cups.

 * Volumes with no holes at the top (model +Z). It's desirable for as much
   resin as possible to drip free from the print before moving to the
   cleaning and curing stages.

   By adding a single inclined hole at the back face, resin is allowed
   to drain out of the central area once the solid layers of the key
   are reached

These are enabled by the existing `module resin()`, so the typical use
remains: `resin() key()`.

This placement works with all standard cherry row profiles.