Add warning for invalid footnotes reference (and unused definitions)

[asmeurer]

What version of myst-parser are you using?

3.0.1

What version dependencies are you using?

Sphinx 7.2.6 docutils 0.20.1

What operating system are you using?

Mac

Describe the Bug

If a Markdown footnote does not have a corresponding reference in either direction, no warnings are emitted. Compare the equivalent RST and Markdown:

Lorem ipsum [#f1]_ dolor sit amet ... [#f2]_

.. rubric:: Footnotes

.. [#f1] Text of the first footnote.
.. [#f3] Text of the second footnote.

Lorem ipsum [^f1] dolor sit amet ... [^f2]

```{rubric} Footnotes
```

[^f1]: Text of the first footnote.

[^f3]: Text of the second footnote.

The first one gives two warnings in the build

test.rst:1: ERROR: Too many autonumbered footnote references: only 0 corresponding footnotes available.
test.rst:1: ERROR: Unknown target name: "f2".

The second one gives no warnings. The resulting document just has a literal [^f2] in the text.

Expected Behavior

An invalid footnote should produce a warning, ideally in both directions, since it is an invalid cross-reference.

To Reproduce

No response