Underline Integration

Hey, thanks for your work! Could you maybe integrate the possibility to add underlines instead of highlights? Incl. an obsidian command for it, the color black or a free choice of colors? That would be great for my needs. ;)

Start by locating the existing PDF highlight integration and the Obsidian command handling. Implement underline annotations with black as the default and support a free color choice; done means users can trigger underlining from Obsidian and see the annotation in the saved PDF.

Written by the indexing model from the issue text.