gh-76535: Add C API functions for changing case of a single codepoint

[lysnikolaou]

Additionally, move iscased and iscaseignorable from the internal API to the public API and add convenience macros. These are needed to handle the capital sigma case.

Closes #76535.

• Issue: gh-76535

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📚 Documentation preview 📚: https://cpython-previews--117117.org.readthedocs.build/

[serhiy-storchaka]

Please add tests. I see a handful of flaws in the design and the implementation, the tests will expose them.

[lysnikolaou]

Closing this in favor of #136176.