Unite "cite" and "cite this volume" commands

[kopeckyf]

Currently, we have the usual \cite commands, but also additional \citetv commands that output "[this volume]". It would be desirable to have only one type of commands that automatically detects whether the given cite key refers to a chapter in the volume or a regular bib resource. Since all items cited in this volume are prefixed with chapters/ or suffixed with .tex, an implementation in langsci-unified.cbx should be pretty straight-forward.