Enable polymode within code-blocks in Markdown files. Effectively, it
allows code-blocks of various languages to be handled by their
respective major modes, allowing correct syntax highlighting, snippets,
and other features to work as you would expect.
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.
