feat: allow sending toast notifications

[celeriev]

Enable sending toast notification from chainlit app

Related to #1639

# This is a simple example of a chainlit app.

from chainlit import AskUserMessage, Message, on_chat_start
import chainlit as cl

@on_chat_start
async def main():
    res = await AskUserMessage(content="What is your name?", timeout=30).send()
    if res:
        await cl.context.emitter.send_toast(message=f"Hello {res['output']}!")
        await cl.context.emitter.send_toast(message=f"Hello {res['output']}!", type="error")

This is my first PR 🥳

[willydouhard]

This is looking great! I think we could add a function on the emitter class with typing on top of cl.context.emitter.emit("toast" to make it more discoverable and usable.

[celeriev]

@willydouhard I added the richColors settings, let me know what you think, I think it goes quite well with chainlit look

[hexart]

How is this pr going? Can I make a new request as @willydouhard asked for?

Related issue #1797

[celeriev]

It is ready I think to be merged, wdyt @willydouhard @dokterbob

[willydouhard]

LGTM