feat: introduce mode_any aggregation

[MarcoGorelli]

We currently have Expr.mode and Series.mode, like Polars does. What's not great about them is that they don't aggregate

I'd like to suggest a departure from Polars here. We introduce Expr.mode_any and Series.mode_any, which actually aggregate, but make no guarantees about which mode will be returned. For example, if a column contains values [1, 2, 2, 3, 3, 4], then it may return 2 or it may return 3.

[MarcoGorelli]

Alternatively, per #1657, if we disallow length-changing-non-aggregating expressions which aren't followed by aggregations, then we might be able to get away with just Expr.mode, and just require that it be followed by an aggregation, .e. nw.col('a').mode().first(order_by='id'). We could even introduce Expr.any_value and allow nw.col('a').mode().any_value()

[FBruzzesi]

Returning a list would definitly be consistent with polars mode behavior in a group by context. We might always return a list type (and enhance a bit the list namespace with .first, .last, .__getitem__)

[MarcoGorelli]

I'm not sold on returning lists here for expressions which don't aggregate, and I think Polars is a bit internally inconsistent here. .list is also a bit problematic for some backends, like pandas non-pyarrow-backed, whereas with nw.col('a').mode().any_value() / nw.col('a').mode().first(order_by='id') we can bypass the list and have a return value which works well for all implementations