[New plugin] pandoc-crossref

[sys9kdr]

Is your feature request related to a problem? Please describe

it would be nice to have support for pandoc-crossref

Describe the proposed solution

I created a plugin https://github.com/sys9kdr/asdf-pandoc-crossref

Describe similar asdf features and why they are not sufficient

new plugin

Describe other workarounds you've considered

new plugin

[sys9kdr]

see #997