feat(archive/100-theorems-list): Partition theorem (#4259)

A proof of Euler's partition theorem, from the Freek list.
The proof is sorry-free but currently unpleasant, and some parts don't belong in `archive/`, so WIP for now.

Co-authored-by: Ruben Van de Velde <ruben.vandevelde+github@gmail.com>
Co-authored-by: Aaron Anderson <awainverse@gmail.com>

Co-authored-by: Julian Berman <Julian@GrayVines.com>
Co-authored-by: Ruben Van de Velde <65514131+Ruben-VandeVelde@users.noreply.github.com>
Co-authored-by: Floris van Doorn <fpvdoorn@gmail.com>