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.

modules/languages/siren-php.el

@@ -7,7 +7,6 @@
 ;;; Code:
 
 (require 'siren-folding)
-(require 'siren-highlight-indentation)
 (require 'siren-rainbow)
 
 (use-package php-mode
@@ -21,7 +20,6 @@
     (rainbow-mode +1)
     (company-mode +1)
     (subword-mode +1)
-    (highlight-indentation-current-column-mode)
     (hs-minor-mode 1)
     (hideshowvis-enable)))