Bind the correct this context to chat's callback methods passed down to <markdown-stream>

[cpsievert]

Follow up to #1845.

Note that, before this change, running the example below results in the following JS console warning:

markdown-stream.ts:122 Failed to call onContentUpdate callback: TypeError: Receiver must be an instance of class anonymous

from shiny.express import ui

welcome = """
<p>Here is the <span id="first" class='suggestion'>1st input suggestion</span>.
And here is a <span id="second" data-suggestion='The actual suggestion'>2nd suggestion</span>.
Finally, a <img id="third" data-suggestion='A 3rd, image-based, suggestion' src='shiny-hex.svg' height="50px" alt='Shiny logo'> image suggestion.</p>

<p>On the other hand, <span id="fourth" class="suggestion submit">this suggestion will auto-submit</span>.
And <span id="fifth" data-suggestion="another suggestion" data-suggestion-submit="true">this suggestion will also auto-submit</span>.</p>
"""

chat = ui.Chat(
    "chat",
    messages=[welcome],
)


chat.ui()


@chat.on_user_submit
async def on_user_submit(message: str):
    await chat.append_message(f"You said: {message}")