Add markdown_stream()

[cpsievert]

In anticipation of posit-dev/py-shiny#1782, which adds ui.MarkdownStream() to Shiny for Python

• 83c0f3d does a minimal change of just pulling in the new markdown-stream JS/CSS assets that the chat component will now depend upon.
• 9f36cb0 takes a first pass at adding markdown_stream() and output_markdown_stream().

Open questions

• Is shinychat a proper home for markdown_stream()?
• How best to handle mid-stream errors? Show them in a notification? Should they crash the app?

TODO

• Add height/width to output_markdown_stream()?
• Get Add ui.MarkdownStream py-shiny#1782 merged before merging this

[cpsievert]

@jcheng5 I'm gonna merge before a review since I'd like to demo it tomorrow, but please feel free to review and I'll address in a follow up PR