[Enh]: Support `Expr.len()`

[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?

No response

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

nw.len() is not enough is some situation. Here a reproducible example in polars:

import polars as pl

df = pl.DataFrame({"a": [-1.5, -.5, 0.5, 1.5, 2.5], "b": [1,2,1,2,1,]})
df.select(
    pl.col("a").filter(pl.col("b")==1).len().alias("1"),
    pl.col("a").filter(pl.col("b")==2).len().alias("2"),
)

shape: (1, 2)
┌─────┬─────┐
│ 1   ┆ 2   │
│ --- ┆ --- │
│ u32 ┆ u32 │
╞═════╪═════╡
│ 3   ┆ 2   │
└─────┴─────┘

Suggest a solution if possible.

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If you have tried alternatives, please describe them below.

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Additional information that may help us understand your needs.

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