cad/libbgcode: new port

It is required by PrusaSlicer 2.7+. This port provides library and
binary to work with g-code (read/write/convert)

Reported by: Teodor Sigaev <teodorsigaev@gmail.com>
Reviewed by: lwhsu (mentor)
Sponsored by: Postgres Professional
Differential Revision: https://reviews.freebsd.org/D44257