Missing fields in P3 serializer

[stephenwf]

• Content resource linking properties (e.g. seeAlso)
• Annotation linking + descriptive properties
• Agent linking properties

The output ends up like:
https://codesandbox.io/s/iiif-builder-forked-rd8b6q?file=/src/index.js

Where __$UNSET$__ is used instead of the linked property.

[mathewjordan]

Thanks for noting this. This is something I also noticed when adding homepage property within a partOf entry in iiif-builder. Had not traced it down to parser at that point, but it makes sense now