feat(linear_algebra/linear_pmap): add has_smul and ext (#14915)
Adds the type-class `has_smul` for partially defined linear maps. We proof the ext lemma.
Co-authored-by: Moritz Doll <doll@uni-bremen.de>
Co-authored-by: Yury G. Kudryashov <urkud@urkud.name>
