Add `ui.MarkdownStream` #6137

Job	Run time
playwright-deploys-precheck (3.10)	5m 46s
check (3.12, ubuntu-latest)	2m 47s
test-narwhals-integration	42s
playwright-examples (3.12, chromium)	3m 52s
playwright-shiny (3.12, chromium)	4m 49s
Deploy to PyPI	0s
	17m 56s

Provide feedback