Update for pandoc 2.16

[bpj]

Pandoc has changed the Lua type of its objects from table to userdata, which necessitates some changes in this filter. I'll probably push a fix in the first week of 2022.

[dsp77]

Will this change be backward compatible with older versions of pandoc?

[bpj]

That's the idea, and I think I have come up with a way to do it. We'll soon see if it works.