Org reader: allow comments above property drawer

The Org Manual page at https://orgmode.org/manual/Property-Syntax.html
says (as of 2022-02-03):

"Property blocks defined before first headline needs to be located at
the top of the buffer, allowing only comments above."

This commit allows comments above.

src/Text/Pandoc/Readers/Org/DocumentTree.hs

@@ -41,6 +41,7 @@ documentTree :: PandocMonad m
              -> OrgParser m (F Inlines)
              -> OrgParser m (F Headline)
 documentTree blocks inline = do
+  many commentLine
   properties <- option mempty propertiesDrawer
   initialBlocks <- blocks
   headlines <- sequence <$> manyTill (headline blocks inline 1) eof
@@ -59,6 +60,9 @@ documentTree blocks inline = do
       , headlineContents = initialBlocks'
       , headlineChildren = headlines'
       }
+  where
+    commentLine :: Monad m => OrgParser m ()
+    commentLine = commentLineStart <* anyLine
 
 -- | Create a tag containing the given string.
 toTag :: Text -> Tag