feat: Add chat_app(), chat_mod_ui() and chat_mod_server()

[gadenbuie]

For tidyverse/ellmer#397

• Moves the implementation of ellmer::live_browser() to shinychat as shinychat::chat_app().

• Adds a simple Shiny module, that can be initialized from a chat client or can be intialized in the same way as chat_ui(). The server-side logic connects the user input and wraps client$stream_async() |> chat_append().

The module also uses an ExtendedTask to avoid blocking reactivity; the docs include an example with simultaneous Claude/OpenAI chats that can run in parallel.

[cpsievert]

LGTM, but would also be great to have @jcheng5's take, especially in regard to ExtendedTask and whether he'd be open to chat_append() using it (since it seems like it'll be necessary for prompting the user for input during tool execution)