Summary

This PR adds comprehensive tests to prove Unicode support across all functionality of the library, addressing issue #156.

Changes

• Unicode grammar parsing: Tests that grammars with Unicode terminals and nonterminals parse correctly
• Unicode sentence generation: Tests that random sentence generation works with Unicode content
• Unicode input parsing: Tests that input strings with Unicode characters can be parsed against grammars
• Unicode parse tree display: Tests that parse trees display Unicode characters correctly
• Unicode parse tree iteration: Tests that parse tree iteration works with Unicode content
• Unicode Mermaid formatting: Tests that parse trees can be formatted as Mermaid diagrams with Unicode
• Unicode grammar serialization: Tests that Unicode grammars can be converted to string and back
• Complex Unicode grammars: Tests that mixed Unicode/ASCII grammars work correctly

Testing

• ✅ All 94 tests pass
• ✅ Code formatted with
• ✅ All clippy checks pass with
• ✅ No breaking changes to existing functionality

Issue Reference

Closes #156 - Unicode support documentation and testing

Background

As noted in issue #156, there was some confusion about whether the crate supported Unicode. These tests provide concrete proof that Unicode is supported throughout the library's functionality, which should help clarify this for future contributors.