Add native PowerPoint comments for slide elements

[Osuna0102]

Summary

• add native PowerPoint comments for text, shapes, tables, and images
• generate modern comment parts, author metadata, and element-attached markers
• document usage in the README and add a node demo for comments

Validation

• built the library with npm run build
• regenerated distribution files with npm run ship
• generated a demo presentation with commented elements
• opened the generated file in Microsoft PowerPoint and verified the modern comment structure survives round-trip save

[Osuna0102]

Hello!

I basically just added this pull request because I needed to add "references" of the data for each element added in my PPTX files, instead of a reference slide at the end.

Added a little demo file to test the output, and added the documentation with the usage like the official docs.

IDK if there is another use cases for this, but you find it useful feel free to merge it.

Have a great day :)