Pandoc 2.19 has deprecated --self-contained

[dmurdoch]

saveWidget() calls pandoc_self_contained_html() which calls Pandoc with the --self-contained option. However, Pandoc 2.19 has deprecated that option in favour of --embed-resources --standalone, and so it gives a warning. For example, see https://stackoverflow.com/questions/74379298/argument-selfcontained-deprecated-in-htmlwidgetssavewidget .

[cpsievert]

This has already been addressed via #443