Add messages parameter to chat_ui() and Chat.ui(); revert #1593

[cpsievert]

With this PR, the messages parameter of Chat.ui()/chat_ui() becomes the suggested way to provide starting messages. For example:

Express

from shiny import reactive
from shiny.express import ui

chat = ui.Chat(id="chat")

chat.ui(messages=["Hello, how can I help?"])

Core

from shiny import App, ui

app_ui = ui.page_fixed(
    ui.chat_ui("chat", messages=["Hello, how are you?"]),
)

def server(input, output, session):
    pass

app = App(app_ui, server)

This PR also reverts #1593 since waiting until flush to send is not an ideal way to support dynamic UI. As a result, this PR also closes #1735.

In the next release, we should deprecate Chat(messages=...) in favor of these new parameters. Deprecating now feels premature since it's still convenient for things like system messages. But, when we move towards recommending chatlas/LangChain for managing chat state in a future release, we'll deprecate as a part of that move.