The installer now depends on rich, a Python library that makes it

easier to handle color, emoji, and layout in text-based applications.
We build upon those primitives, adding light/dark themes,

non-interactive prompt handling, headers, admonitions, multi-paragraph

display, terms, key/value display, ordered lists, and our own defaults

for progress bars.
This will be used by the upcoming install wizard.
Some of this may later be reworked into a shared library, for use in

both rbinstall and rb-site.

Made extensive use of this in the wizard.
As this is purely visual work, there's no unit tests for this module.