feat: add `Series|Expr.cum_count` method #2653

Job	Run time
altair (3.12, ubuntu-latest)	1m 59s
marimo (3.12, ubuntu-latest, core,optional)	2m 0s
scikit-lego (3.11, ubuntu-latest)	2m 0s
shiny (3.12, ubuntu-latest)	3m 25s
tubular (3.12, ubuntu-latest)	54s
	10m 18s

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