Add `ui.MarkdownStream` #6188

Job	Run time
check (3.12, ubuntu-latest)	3m 16s
check (3.12, windows-latest)	4m 41s
playwright-shiny (3.12, chromium)	4m 44s
check (3.12, macOS-latest)	2m 32s
playwright-shiny (3.12, firefox)	5m 28s
playwright-deploys-precheck (3.10)	1m 50s
check (3.11, ubuntu-latest)	2m 34s
playwright-shiny (3.12, webkit)	4m 40s
check (3.11, windows-latest)	5m 49s
playwright-shiny (3.11, chromium)	4m 20s
check (3.11, macOS-latest)	2m 38s
test-narwhals-integration	3m 52s
playwright-shiny (3.11, firefox)	4m 49s
check (3.10, ubuntu-latest)	2m 35s
playwright-shiny (3.11, webkit)	4m 37s
check (3.10, windows-latest)	5m 11s
playwright-examples (3.12, chromium)	3m 53s
playwright-shiny (3.10, chromium)	4m 23s
check (3.10, macOS-latest)	2m 28s
playwright-examples (3.12, firefox)	4m 22s
playwright-shiny (3.10, firefox)	5m 34s
check (3.9, ubuntu-latest)	2m 39s
playwright-examples (3.12, webkit)	4m 16s
playwright-shiny (3.10, webkit)	12m 25s
check (3.9, windows-latest)	3m 36s
playwright-examples (3.11, chromium)	3m 43s
playwright-examples (3.11, firefox)	5m 25s
check (3.9, macOS-latest)	2m 42s
playwright-shiny (3.9, chromium)	6m 50s
playwright-examples (3.11, webkit)	3m 49s
playwright-examples (3.10, chromium)	3m 44s
playwright-shiny (3.9, firefox)	5m 42s
playwright-examples (3.10, firefox)	4m 42s
playwright-examples (3.10, webkit)	4m 0s
playwright-examples (3.9, chromium)	3m 51s
playwright-examples (3.9, firefox)	4m 18s
playwright-examples (3.9, webkit)	3m 59s
Deploy to PyPI	0s
	2h 39m 57s

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