Fix pstyle serialization within Level

[git-noise]

What does this change?

Fix ParagraphStyle handling within a Level. Attempts to fix https://github.com/bokuweb/docx-rs/issues/737. This focuses on the rust-side of the code - i.e docx-core - but docx-wasm tests seem to pass.

References

• https://github.com/bokuweb/docx-rs/issues/737.
• https://learn.microsoft.com/en-us/dotnet/api/documentformat.openxml.wordprocessing.paragraphstyleidinlevel?view=openxml-3.0.1
• http://officeopenxml.com/WPnumberingLvl.php

Screenshots

Not applicable

What can I check for bug fixes?

Not applicable