feat: kurtosis expression

[raisadz]

Is your feature request related to a problem?

Ibis support in Narwhals narwhals-dev/narwhals#2000

What is the motivation behind your request?

No response

Describe the solution you'd like

ibis._['a'].kurtosis()

Kurtosis calculated with the Fisher's definition without bias correction.

What version of ibis are you running?

10.5

What backend(s) are you using, if any?

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[cpcloud]

Thanks for the report! Is there a particular backend you're interested in?

[raisadz]

Thank you for the reply @cpcloud. I have a preference for the DuckDB backend but I wouldn't mind if any other backend will be implemented first so that kurtosis becomes a part of the ibis's API.