[Enh]: cumulative features

[FBruzzesi]

We would like to learn about your use case. For example, if this feature is needed to adopt Narwhals in an open source project, could you please enter the link to it below?

Cumulative features, together with forward fill and some other checks/hacks, would most likely be enough to enable the equivalent of pandas expanding operations. This is a requirement to complete plotly/plotly.py#4834.

Please describe the purpose of the new feature or describe the problem to solve.

List of cumulative expressions supported by polars:

• cum_count
• cum_min
• cum_max
• cum_prod
• cum_sum

With these, we would enable the following additional univariate expanding operations: mean, var, std, skew, kurt.

What's left out is: median, quantile, sem, rank (entire pandas expanding window function list).