Quotes and brackets as multiparagraph divisions

This commit introduces a new concept of multiparagraph divisions (where
the lines within are re·analysed after the prefix is removed) and
transitions block quotations to it. I resisted this for a long while
because it’s less “simple”, but the precedent of preformatted text and
the old quotation mechanism (which was kind of messy) was already
conceptually close. And footer support really requires this.