Also create a new siren-display-line-numbers module and function that's
responsible for turning on the display of line numbers. Future changes
to how line numbers are displayed can now be done in a single place.
- Switch from highlight-indentation package to highlight-indent-guides.
- Activate visual indentation in a prog-mode hook, rather than doing
within each individual major mode. It was already done within all
major modes based on prog-mode anyway.
- Add new siren-display-indetation module and function as a central way
to enable visual indetation guides. This makes switching the
underlying package at some point in the future much easier.
