refactor: Visual indentation guides

- 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.

core/siren-core-modules.el

@@ -22,6 +22,7 @@
 
 ;; Editor
 (require 'siren-browse-kill-ring)
+(require 'siren-display-indentation)
 (require 'siren-helm-command)
 (require 'siren-ido)
 (require 'siren-linum)