feat(Chat): The .append_message() method now automatically streams in generators

[cpsievert]

Most importantly, this'll make it so .append_message() "just works" with chatlas' .stream() method

from chatlas import ChatAnthropic
from shiny.express import ui

chat = ui.Chat(
    id="ui_chat",
    messages=["Hi! How can I help you today?"],
)
chat.ui()

chat_model = ChatAnthropic()

@chat.on_user_submit
async def _():
    response = chat_model.stream(chat.user_input())
    await chat.append_message(response)

TODO

• Unit test