feat: dataframe download separator #20261

Job	Run time
changes	9s
Tests examples and smoke_tests	19s
Build marimo wheel	1m 23s
Test Pyodide	29s
Tests CLI on macos-14 3.12	3m 25s
Tests CLI on ubuntu-latest 3.12	4m 6s
Tests CLI on windows-latest 3.12	8m 19s
	18m 10s

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