Extend chatfeed card stylesheets through card_params

[pmeier]

#6154 added the ability to pass parameters to the Card though the pn.chat.ChatFeed:

panel/panel/chat/feed.py

Lines 155 to 156 in 00a45d7

	card_params = param.Dict(default={}, doc="""
	Params to pass to Card, like `header`, `header_background`, `header_color`, etc.""")

Unfortunately, it is impossible to send additional stylesheets, because the builtin ones are simply overwritten by whatever one passes through the chat feed:

panel/panel/chat/feed.py

Lines 270 to 272 in 00a45d7

	stylesheets=self._stylesheets + self.param.stylesheets.rx(),
	)
	card_params.update(self.card_params)

To achieve that, one has to access the private attribute:

chat_feed._card.stylesheets.extend([...])

This is what we do over at Ragna.

Would it be possible to refactor how card_params are used internally to update the stylesheets rather than flat-out overwriting them?