feat(Chat): A .on_user_submit callback can now access user input through a function parameter

[cpsievert]

This way you can get the user input directly from the submit callback rather than the chat.user_input() 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 handle_user_input(user_input):
    response = chat_model.stream(user_input)
    await chat.append_message_stream(response)

Also, looking forward, a decent way to provide access to uploaded files/images might be through additional function parameters?