Add footnote attributes that mirror cmark-gfm

[digitalmoksha]

This aligns the footnote output with cmark-gfm. Relates to #259

[kivikakk]

Hi @digitalmoksha — thanks so much for this, this is great.

The spec builds are failing, but that's only because the extensions.txt spec in our vendored cmark-gfm doesn't match the new output. I tried pulling in the current upstream for that part of the file, and it also doesn't quite work due to some remaining differences in footnote ID generation (and a parsing issue with footnotes like Hello[^"><script>alert(1)</script>], which are apparently supported!).

That's fine with me — I'll address it in a follow-up PR. This here is great.

[digitalmoksha]

Thanks @kivikakk for getting this over the line 🚀