[epic]: Bookmarking

[schloerke]

TODO - Barret

• R-shiny Pass at ellmer / shiny integrations: feat: Add chat_enable_bookmarking(id, client, ..., update_on_input = TRUE, update_on_response = TRUE) shinychat#28
• py-shiny bookmark hooks (see above comment for examples)
• py-shiny input integration w/ bookmark hooks; Ex: restoreInput() method
• chatlas integration (see above comment for example)
• py-shiny App UI. Upgrade to a function.
• py-shiny express UI integration. UI is currently pre-processed. It needs to be upgraded to a function

Goal: < 2 weeks from now