allow `line breaks` to be defined in rules

yaml does not support line breaks. However, sometimes we need manual line breaks with rules.

Consider using `@` to represent the line breaks in rules.

update: in footnotes we are already using `|` as line breaks. It might be good to still use this as the line breaks

Question for @Melkiades and @shajoezhu :

is manual line breaks in header/row labels/content causing issues or not?