mathlib

1e89df22 - chore(algebra/monoid_algebra): convert a filename prefix into a folder (#8925)

chore(algebra/monoid_algebra): convert a filename prefix into a folder (#8925) This moves: * `algebra.monoid_algebra` to `algebra.monoid_algebra.basic` * `algebra.monoid_algebra_to_direct_sum` to `algebra.monoid_algebra.to_direct_sum`