Add highlights to existing file

[kelszo]

It would be nice to be able to sync highlights to an existing file (such as notes created by Book Search). Initially, I considered using the ISBNs from the SQLite database and matching them with an ISBN field in the frontmatter for this purpose. However, the complexity arises due to the nature of ISBNs. Since a single book may have multiple ISBNs, varying with the publisher and edition, this method might result in incorrect identification of many books. A more reliable, albeit slightly unorthodox, approach could be to perform string matching between the book's title/author fields between the SQLite database and frontmatter.

Should this feature seem like a viable addition to the code base, I am happy to contribute by submitting a PR.

[OGKevin]

hmm, I think this feature can use some of the work that has been done in #107

I'll poke the author one more time if they have any plans on finishing it, if not, i'll pick up where they've left off and finish it.
Then we can see if the markers introduced in this file can be used.