The pcb used for wheatfield/split75 supports ANSI and ISO layout. This
commit adds the ISO variant.
With suggestions from code review:
Co-authored-by: Drashna Jael're <drashna@live.com>
Co-authored-by: Joel Challis <git@zvecr.com>
Co-authored-by: James Young <18669334+noroadsleft@users.noreply.github.com>
