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.
Additionally allow `recursive-narrow-or-widen-dwin` to also trigger
`org-edit-src-code` within org buffers. This technically does not use
narrowing, but it does "focus" on the source code block for editing, so
it at least in spirit embodies the intent of narrowing.
This behavior was inspired by:
https://endlessparentheses.com/emacs-narrow-or-widen-dwim.html