auto-complete for footnotes

Given a document defines these footnotes:

```md
[^greenleaf]: Robert Greenleaf: "Essentials of Servant Leadership”, 1977

[^sendjaya-paper]: https://journals.sagepub.com/doi/10.1177/107179190200900205
```

When I type `[^`
Then I want to see these autocomplete entries:

```
^greenleaf
^sendjaya-paper
```