Fix quoted comma assignments

[vamshikrishnaramasamy]

Summary

• parse quoted text assignments with commas as regular assignments from level 4 onward
• keep unquoted comma assignments parsing as lists
• add grammar regression coverage for levels 4 through 12

This fixes hedyorg/hedy#6468 and supersedes hedyorg/hedy#6593 by applying the fix in the source grammar repo.

Test

• uv run pytest tests/test_grammar.py