chore(topology/algebra/module, analysis/normed_space/linear_isometry): dedup `submodule.subtypeL` and `continuous_linear_map.subtype_val` (#15700)
To designate the continuous linear inclusion of a submodule into the ambient space, we currently have both `continuous_linear_map.subtype_val` (correct assumptions, name not consistent with `submodule.subtype`) and `submodule.subtypeL` (good name, but way too strong assumptions). This keeps the best of both worlds.