Add `theme_picker_{ui,server}()` functions

[schloerke]

Fixes #11

Thank you @vnijs for the start in #11

TODO:

• [x] Changelog entry
• [x] docs
• [x] quartodoc entry
• [x] readme blurb

https://github.com/rstudio/py-shinyswatch/assets/93231/eb24853b-4545-4558-8f11-3921bc3347a5

[schloerke]

I have it as regular UI / Server functions and a not to have it in a module. I think it's fair and simplifies the API.