This allows editing code blocks within markdown files in a separate
buffer with the proper language major-mode applied.
Also integrate it with recursive-narrow, so it can be triggered with via
narrowing commands.
This module wraps around the fill-column-indicator and it's
fci-mode. This will enable easy switch-over to Emacs 27's native
display-fill-column-indicator feature.
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.