Add a DocumentRoot context element storing the original full document.

This might be used for look-behind instead of storing previous element nodes in the context tree.

src/parser/parser_context.rs

@@ -94,6 +94,7 @@ impl<'r, 's> ContextTree<'r, 's> {
 
 #[derive(Debug)]
 pub enum ContextElement<'r, 's> {
+    DocumentRoot(&'s str),
     ExitMatcherNode(ExitMatcherNode<'r>),
     PreviousElementNode(PreviousElementNode<'s>),
     Context(&'r str),