Add ability to update Chat()'s input placeholder

[cpsievert]

Via a new update_user_input() method. As a result, set_user_message() is now deprecated. Here's an example of using it to update the input's placeholder:

from shiny import App, module, reactive, ui


@module.ui
def chat_module_ui():
    return [
        ui.chat_ui("chat"),
        ui.input_action_button(id="placeholder_increment", label="Increment"),
    ]


@module.server
def chat_module_server(input, output, session):
    chat = ui.Chat(id="chat")

    @reactive.effect
    @reactive.event(input.placeholder_increment)
    def _():
        chat.update_user_input(
            placeholder=f"placeholder {input.placeholder_increment()}"
        )

    @chat.on_user_submit
    async def submit_chat():
        user = chat.user_input()
        await chat.append_message(f"You said: {user}")


app_ui = ui.page_fillable(
    ui.panel_title("Hello Shiny Chat"),
    chat_module_ui("chat_container"),
    fillable_mobile=True,
)

def server(input, output, session):
    chat_module_server("chat_container")

app = App(app_ui, server)