Move the indent level for plain list's exit matcher to const fn instead of grabbing from the context.

This made a slight improvement to performance.

src/parser/parser_context.rs

@@ -112,9 +112,6 @@ pub enum ContextElement<'r, 's> {
     ExitMatcherNode(ExitMatcherNode<'r>),
     Context(&'r str),
 
-    /// Stores the indentation level of the current list item.
-    ListItem(usize),
-
     /// Stores the name of the greater block.
     GreaterBlock(&'s str),