updates remark/rehype/unified to the latest versions, and reconfigures
the markdown pipeline to be more efficient and use the latest plugins.

the pipeline now includes GFM style features like code highlighting,
footnotes, inline urls, and hard line breaks, and no longer double
parses to html.

also implements the first part of code highlighting in markdown,
what's left is correctly bringing in theme css files

the full theme is loaded, but controled by a parent's data attribute.

p>div is not a valid nesting in HTML, and with strict-mode turned on,
react complains about it.

this fix allows labels to be inline, and then uses that in the LabelChange
component to avoid getting hydration warnings from react