`convert` handles markdown footnotes, but `build` does not

[digitalronin]

Given an empty directory out, and in/index.mmd containing:

# Test

Here is some text referencing footnote one[^1].

[^1]: This is a footnote.

Then running npx mpx build --input in --output out does not handle the footnote correctly. You end up with html that renders like this in a browser:

Here is some text referencing footnote one[^1].
[^1]: This is a footnote.

(where the text [^1] is just plain text, not a link)

However, npx mpx convert in/index.mmd out/index.html converts the footnote correctly, so that you end up with html that renders like this:

Here is some text referencing footnote one[1].
This is a footnote. ↩︎

(where the [1] is a link to the footnote, and the ↩︎ is a link back to the relevant point in the document)