allow `line breaks` to be defined in rules

[clarkliming]

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?

[Melkiades]

It is not simple, but it should be supported to some extent. The rendering machinery at the moment does not dial too well with its structure and detecting \n is done at rendering time and should somehow communicate before with the rendering structure (e.g. if there is a newline in a column label to assign the number of lines of the header/column label to 2). I would need a bit of time to reassess the current situation of the rendering with \n

what do you mean with @?

[clarkliming]

what do you mean with @?

because we can not define "line breaks" in yaml, so we have to use some symbols, like @ to represent the line breaks (after reading data from yaml)

[clarkliming]

@crazycatandy it is not completed

[clarkliming]

we either move it to later PI, or open new issues to track