Add `NestedSeries.len()`

[hombit]

We already have NestedSeries.list_lengths/NestedAccessor.list_lengths attributes, but probably the name is not clear enough. Pandas has .list.len() inside the list accessor, so I propose to use the same API.

[scharney]

I agree! I think it would be useful and more easily understandable a priori if the attributes were named the same thing,