Add support for bookmarks in the style file

This commit introduces support for bookmarks in the style file. It
includes changes in the `style.css` file to add `-prince-bookmark-level`
and `-prince-bookmark-label` properties to the `book` and `chapter`
elements, allowing for better organization and navigation within the
document.

Author: placek

style.css

@@ -14,8 +14,8 @@
 body { font-size: var(--size); text-align: justify; font-family: greek, mono, f1, f2; line-height: 1.2; }
 h1 { font-family: mono; font-weight: 600; font-size: 6vh; margin: 46vh 0; padding: 0; text-align: center; }
 info { display: flex; width: 100%; justify-content: space-between; }
-book { string-set: book attr(data-book); display: block; break-before: left; page: book; }
-chapter { string-set: chapter attr(data-chapter); display: flex; }
+book { string-set: book attr(data-book); display: block; break-before: left; page: book; -prince-bookmark-level: 1; -prince-bookmark-label: attr(data-book); }
+chapter { string-set: chapter attr(data-chapter); display: flex; -prince-bookmark-level: 2; -prince-bookmark-label: attr(data-chapter); }
 chapter:before { content: attr(data-chapter); font-family: mono; font-weight: 600; }
 column { display: block; padding: 0 .5em; }
 verse { margin: 0; padding: 0; display: inline; }