Implement a custom pandoc Lua filter for .Rmd -> .MyST conversion

[mpadge]

Because rmarkdown::md_document() only offers a few hard-code options, none of which are MyST-compatible. That function is nevertheless extensible via https://pandoc.org/lua-filters.html

[mpadge]

Here is the relevant issue in the MyST repo: executablebooks/rst2myst#2