Wrap Google-style Args and Returns entries without flattening the section

[akul-ameya]

Summary

• Fixes wrapping around Google-style Args/Arguments and Returns sections (issue Wrapping doesn't work at all when there is an argument list? #174).
• Unlike Fix issue #174: Enable line wrapping in docstrings with argument lists while preserving section formatting #324, this wraps each field entry independently instead of preserving the whole section unchanged.
• Reflows already-wrapped entries and is idempotent: format(format(source)) == format(source).

Test plan

• poetry run pytest tests/formatter/test_google_field_wrapping.py tests/patterns/test_google_sections.py
• poetry run pytest tests/formatter tests/patterns tests/wrappers
• poetry run pytest (Linux CI; local Windows e2e looks for VIRTUAL_ENV/bin/docformatter)