mathlib3

a176cb12 - feat(linear_algebra/matrix/schur_complement): invertibility of block matrices (#19156)

feat(linear_algebra/matrix/schur_complement): invertibility of block matrices (#19156) Co-authored-by: Jeremy Tan Jie Rui <e0191785@u.nus.edu> Co-authored-by: Jireh Loreaux <loreaujy@gmail.com>