Add `ui.MarkdownStream` #6171

Job	Run time
check (3.12, ubuntu-latest)	2m 48s
check (3.12, windows-latest)	4m 36s
check (3.12, macOS-latest)	1m 53s
check (3.11, ubuntu-latest)	2m 35s
check (3.11, windows-latest)	4m 52s
playwright-deploys-precheck (3.10)	1m 29s
check (3.11, macOS-latest)	2m 23s
check (3.10, ubuntu-latest)	2m 37s
check (3.10, windows-latest)	5m 4s
test-narwhals-integration	4m 14s
check (3.10, macOS-latest)	2m 16s
check (3.9, ubuntu-latest)	2m 2s
check (3.9, windows-latest)	3m 42s
playwright-examples (3.12, chromium)	3m 44s
check (3.9, macOS-latest)	1m 54s
playwright-examples (3.12, firefox)	4m 13s
playwright-shiny (3.12, chromium)	4m 46s
playwright-examples (3.12, webkit)	4m 5s
playwright-shiny (3.12, firefox)	5m 35s
playwright-examples (3.11, chromium)	3m 34s
playwright-shiny (3.12, webkit)	5m 7s
playwright-examples (3.11, firefox)	4m 4s
playwright-shiny (3.11, chromium)	4m 57s
playwright-examples (3.11, webkit)	4m 0s
playwright-shiny (3.11, firefox)	4m 56s
playwright-examples (3.10, chromium)	4m 3s
playwright-shiny (3.11, webkit)	4m 39s
playwright-examples (3.10, firefox)	4m 11s
playwright-shiny (3.10, chromium)	4m 41s
playwright-examples (3.10, webkit)	4m 2s
playwright-shiny (3.10, firefox)	5m 21s
playwright-examples (3.9, chromium)	1m 29s
playwright-shiny (3.10, webkit)	5m 5s
playwright-examples (3.9, firefox)	1m 29s
playwright-shiny (3.9, chromium)	1m 46s
playwright-examples (3.9, webkit)	1m 14s
playwright-shiny (3.9, firefox)	1m 41s
Deploy to PyPI	0s
	2h 11m 7s

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