Split GreaterBlock into CenterBlock, QuoteBlock, and SpecialBlock.

Center and quote blocks do not have parameters nor do they store their name so I am separating them out.

src/parser/element_parser.rs

@@ -81,7 +81,7 @@ fn _element<'b, 'g, 'r, 's>(
     let (remaining, mut affiliated_keywords) = many0(affiliated_keyword_matcher)(input)?;
     let (remaining, mut element) = match alt((
         map(plain_list_matcher, Element::PlainList),
-        map(greater_block_matcher, Element::GreaterBlock),
+        greater_block_matcher,
         map(dynamic_block_matcher, Element::DynamicBlock),
         map(footnote_definition_matcher, Element::FootnoteDefinition),
         map(comment_matcher, Element::Comment),