[Enhancement] Support parsing of multi-line highlights in Markdown importer

The current Markdown importer is unable to correctly parse highlights that contain line breaks. While single-line highlights import without issue, any highlight spanning multiple lines is either ignored, broken into separate fragmented highlights, or causes the import to fail.

This is a significant blocker for users working with complex data. For example, my use case involves language learning where a single "highlight" consists of three distinct lines:

1. The target language text

2. The phonetic pronunciation

3. The translation

Personally, I have a dataset of over 1,700 highlights. Because a large portion of these are multi-line, manual entry or manual correction is not a feasible workaround.