Add warning for invalid footnotes reference (and unused definitions) about myst-parser HOT 3 OPEN

The resulting document just has a literal [^f2] in the text.

Note, a problem with this is it is basically an intended part of commonmark; that square brackets [] are parsed as text, if there are no definitions: commonmark/commonmark-spec#702
footnote references are generally treated in the same way, although perhaps you could make the case that [^ is a more explicit syntax, so that we could remove this "lazy evaluation"; it would be a breaking change for myst, but shouldn't affect too many.

from myst-parser.

Looking at djot, they do indeed always treat [^...] as a footnote, even if no definition is given: https://djot.net/playground/?text=%5B%5Ea%5D&sourcepos=false

(you'll note in https://github.com/jgm/djot, general shortcut link syntax [...] is removed)

from myst-parser.

Thinking about this now actually, perhaps a route to removing the annoying "lazy" behaviour of shortcut link references like [...] for myst, would be to introduce a more "specific" syntax like [=...], e.g.

footnote references are never parsed as text [^footnote]

link shortcuts starting with `[=` are never parsed as text (and the `=` is removed from the link) [=reference]

[^footnote]: This is a footnote

[reference]: https://example.com

goes to:

<p>footnote references are never parsed as text <a id="fnref1" href="#fn1"><sup>1</sup></a></p>
<p>link shortcuts starting with <code>[=</code> are never parsed as text (and the <code>=</code> is removed from the link) <a href="https://example.com">reference</a></p>
<section role="doc-endnotes">
<hr>
<ol>
<li id="fn1">
<p>This is a footnote<a href="#fnref1">↩︎</a></p>
</li>
</ol>
</section>

from myst-parser.