feat(number_theory/cyclotomic/zeta): add lemmas (#11786)
Lemmas about the norm of `ΞΆ - 1`.
From flt-regular.
- [x] depends on: #11941
Co-authored-by: Eric <ericrboidi@gmail.com>
Co-authored-by: Eric Rodriguez <37984851+ericrbg@users.noreply.github.com>
