fix: handle footnotes in link_reference_definition nodes

## Details

Issue: https://github.com/MeanderingProgrammer/render-markdown.nvim/issues/632

Footnotes can appear in
[link_reference_definition](https://spec.commonmark.org/0.31.2/#link-reference-definition)
nodes, we currently do not handle this, hasn't come up before.

Add support by sharing the footnote rendering logic between shortcut
links and link reference definitions. Currently shortcut links can be
many different kinds of things so footnote ends up being the one we try
if nothing else matches. This may need to change in the future if we
render more kinds of shortcut links.