Add optional wrist rest to magic trackpad ergonomic support

This changes the magic_trackpad_ergonomic_support.scad file to include an optional wrist rest.

The `wrist_rest` parameter can be used to enable/disable the wrist rest platform. When set to `no`, the original model is produced.
The `wrist_rest_length` parameter specifies the length of the wrist rest in centimeters.

If you can think of a cleaner way to case on the value of `wrist_rest`, I'd be happy to update this PR.