feat(algebra/quandle): Unital shelves (#17839)
Extend shelves to include the unital case, as well as a few basic results about unital shelves from https://arxiv.org/pdf/1603.08590.pdf
This culminates in providing a monoid instance for a unital shelf.
Co-authored-by: Jim Fowler <fowler@math.osu.edu>
Co-authored-by: Yaël Dillies <yael.dillies@gmail.com>
