Broken footnote links are not included in the broken link callback

When footnotes are enabled, and the footnote link is broken, there doesn't seem to be any indication in the broken link callback.

```console
> cargo run -- -R --enable-footnotes
See [^missing]
```

emits:

```
<p>See [^missing]</p>
```

I would expect these links to be included in the broken link callback.

Currently on fc8fe713f58d7f4495038b48fe76c1f101fb3af1.
