[Enh]: Add support for Series|Expr.is_finite

[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?

Noticed in the inline comment in a Tubular PR

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

Add is_finite methods

Suggest a solution if possible.

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If you have tried alternatives, please describe them below.

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[FBruzzesi]

I have a change in a branch which is 95% ready.
Only discussion needed is how to treat nan's and null's in pandas and its possible backends.

For polars:

import polars as pl

data = {"a": [float("nan"), float("inf"), 2.0, None]}
pl.Series(data["a"]).is_finite()

shape: (4,)
Series: '' [bool]
[
        false
        false
        true
        null
]

For pandas it is hard to distinguish between nulls and nan in a consistent way across all backends