Accept str in pluralization type hints

[Mirochill]

Summary

• Change the public annotations for plural, plural_noun, plural_verb, plural_adj, and singular_noun from Word to str so static type checkers such as Pylance accept ordinary string arguments.
• Keep explicit runtime validation for empty strings, preserving the existing TypeCheckError behavior that Word provided through typeguard.
• Add a NEWS entry for issue Cannot pass str value to plural*() function under type checking of Pylance (reportArgumentType) #233.

Validation

• Not run locally
• Ran git diff --check only; it reported no whitespace errors, only the usual Windows CRLF warning.