Implement the new fields for horizontal rule.

src/parser/horizontal_rule.rs

@@ -38,7 +38,7 @@ where
         space0,
         alt((line_ending, eof)),
     )))(remaining)?;
-    let (remaining, _trailing_ws) =
+    let (remaining, post_blank) =
         maybe_consume_trailing_whitespace_if_not_exiting(context, remaining)?;
     let source = get_consumed(input, remaining);
     Ok((
@@ -49,6 +49,7 @@ where
                 context.get_global_settings(),
                 affiliated_keywords,
             ),
+            post_blank: post_blank.map(Into::<&str>::into),
         },
     ))
 }