Add 18-pin variant of SSOP used by Toshiba (fixes #631)

This is a slight modification of the existing SSOP-20 footprint, with only two pins removed and symmetry restored.

Reference is the TBD62083A datasheet at https://toshiba.semicon-storage.com/info/docget.jsp?did=29893

The linked part is mostly equivalent and pin-compatible to an ULN2804A darlington array, so its schematic symbol can be used.