gh-138135: allow ignoring line breaks in doctest output

[picnixz]

• Issue: Add doctest option for allowing splitting strings without whitespaces #138135

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📚 Documentation preview 📚: https://cpython-previews--138136.org.readthedocs.build/

[picnixz]

This is a proposal but I don't think if it's actually just possible to extend NORMALIZE_WHITESPACE directly as we can't distinguish between a real line break or a visual one (with a line break that is only meant for readability and NORMALIZE_WHITESPACE, we would still end up with a whitespace instead)