Djot reader/writer highlighted text fixes:

- The reader now uses a Span with class "mark" rather than
  "highlighted", for consistency with the other pandoc readers
  and writers.
- The writer renders a Span with sole class "mark" as highlighted
  text.

src/Text/Pandoc/Readers/Djot.hs

@@ -189,7 +189,7 @@ convertInline (D.Node pos attr il) = addAttrToInline pos attr <$>
     D.Str bs -> pure $ str (UTF8.toText bs)
     D.Emph ils -> emph <$> convertInlines ils
     D.Strong ils -> strong <$> convertInlines ils
-    D.Highlight ils -> spanWith ("",["highlighted"],[]) <$> convertInlines ils
+    D.Highlight ils -> spanWith ("",["mark"],[]) <$> convertInlines ils
     D.Insert ils -> spanWith ("",["inserted"],[]) <$> convertInlines ils
     D.Delete ils -> spanWith ("",["deleted"],[]) <$> convertInlines ils
     D.Subscript ils -> subscript <$> convertInlines ils

src/Text/Pandoc/Writers/Djot.hs

@@ -212,6 +212,7 @@ inlineToDjot (Strong ils) = D.strong <$> inlinesToDjot ils
 inlineToDjot (Strikeout ils) = D.delete <$> inlinesToDjot ils
 inlineToDjot (Subscript ils) = D.subscript <$> inlinesToDjot ils
 inlineToDjot (Superscript ils) = D.superscript <$> inlinesToDjot ils
+inlineToDjot (Span ("",["mark"],[]) ils) = D.highlight <$> inlinesToDjot ils
 inlineToDjot (Span attr@(ident,cls,kvs) ils)
   | Just "1" <- lookup "wrapper" kvs
     = fmap (D.addAttr