This helps generalize the use of groups inside zulint.
Introduce list_files to return `by_lang` files dict.
Add feature to create custom groups.
Make custom groups for backend and frontend files.
/bin/sh and /usr/bin/env are the only two binaries that NixOS provides
at a fixed path (outside a buildFHSUserEnv sandbox).
This discussion was split from #11004.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>