enh: improve when-then-otherwise to include chaining

[aivanoved]

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.

As discussed in #588, where a simple when-then-otherwise expression is added, the case for chaining when-then expressions is left to be done at some later point. This will complete and close #47

Suggest a solution if possible.

No response

If you have tried alternatives, please describe them below.

No response

Additional information that may help us understand your needs.

I would be willing to write this code, as looking to complete the open #588 and this is a natural extension

Initial draft proposal can be found at #669