feat: add case function (#447)

[mesejo]

Which issue does this PR close?

Closes #447

What changes are included in this PR?

Besides exposing the case function, this PR also adds the PyCaseBuilder class (to parallel Rust's CaseBuilder)

Are there any user-facing changes?

Yes, but the documentation should be generated automatically

[alamb]

Looks like there is one failing CI check

[jdye64]

Yes, @mesejo there is a check-style test failing if you don't mind fixing. If not I can make the commit to resolve

[mesejo]

Thanks to both of you @alamb and @jdye64 for the review 🙏. The check-style test should be fixed now!

[alamb]

Thanks again @mesejo