Stop stuffing "common" everywhere, shellcheck ftw

Created by: evelikov

The current "common" is fairly misleading - literally half of it is used exclusively within genfstab, plus the in-tree tests. So split it out and use as-needed.

This allows one to reason what is used where - aka is_array() has been unused for years - and addresses some shellcheck warnings. A proper editorconfig and gitlab-CI has been copied from mkinitcpio to aid along the way.

This PR builds on top of https://github.com/archlinux/arch-install-scripts/pull/49 and https://github.com/archlinux/arch-install-scripts/pull/47 - mostly to avoid trivial conflicts.