Instead of requiring a bunch of `<span>` wrangling, add a custom tag,
`<Listing>`, whose attributes get rewritten into the desired output
format, using `<figure>` and `<figcaption>` for improved semantic HTML.

Support both a default (nice and fancy) HTML output mode and a “simple”
mode to make it so we get the desired output format for the NoStarch
input files as well.

This does not yet update all the workspace dependencies to be shared
between the listing and note preprocessors; it *only* integrates the
repo histories.

- Add CSS for `figure.listing`; this now renders identically to how the
  existing listings render.
- Convert the first listing in the book to use `<Listing>` instead of
  a bunch of `<span>`s.