[Request]: Custom blocks with user data that are persistent across imports

[bandantonio]

Feature Request Checklist

• I have searched the existing issues to avoid duplicates

What feature would you like to request?

Different users were mentioning this feature here and there in their reports:

I also built in links to other files for the book, since the importer overwrites any changes to the imported highlights file this gives me somewhere to add thoughts and comments.
@banj in #32

a feature that allowed me to add additional content to the note manually and have these additions persist across imports. As an example, I would love to be able to add sections for my own summary and key concepts of a book
@thorbjaelk in #33

Any book notes I wish to add to inside Obsidian (adding additional notes not present in Apple Books) would have to be moved out of the Apple Books Highlights folder to avoid being overwritten
@mbarritt in #34

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