Theorem conj_act.stabilizer_eq_centralizer

fix(group_theory/subgroup/basic): generalize `centralizer` from `subgroup G` to `set G` (#18965) …

feat(group_theory/group_action/quotient): If `G=<S>`, then `G / Z(G)` embeds into `S → {[g₁, g₂]}` (#17151) …