textproc/ox-gfm.el: new port

This package adds an Org mode export backend for GitHub Flavored
Markdown.

Exporting to Github Flavored Markdown is available through Org mode's
export dispatcher once ox-gfm is loaded. Alternatively, exporting can
be triggered by calling the (autoloaded) function M-x
org-gfm-export-to-markdown.

PR: 275740