Move objects to a lower exit class.

Paragraph's exit matcher which detects elements was causing the plain list parser to exit after the first item was parsed which was causing significant amounts of re-parsing.

src/parser/radio_link.rs

@@ -91,7 +91,7 @@ pub fn radio_target<'r, 's>(
     let (remaining, _opening) = tag("<<<")(input)?;
     let parser_context =
         context.with_additional_node(ContextElement::ExitMatcherNode(ExitMatcherNode {
-            class: ExitClass::Beta,
+            class: ExitClass::Gamma,
             exit_matcher: &radio_target_end,
         }));