Everywhere: Stop using NonnullRefPtrVector

This class had slightly confusing semantics and the added weirdness
doesn't seem worth it just so we can say "." instead of "->" when
iterating over a vector of NNRPs.

This patch replaces NonnullRefPtrVector<T> with Vector<NNRP<T>>.

Userland/Shell/SyntaxHighlighter.cpp

@@ -421,7 +421,7 @@ private:
     {
         for (auto& entry : node->entries()) {
             ScopedValueRollback first_in_command { m_is_first_in_command };
-            entry.visit(*this);
+            entry->visit(*this);
         }
 
         for (auto& position : node->separator_positions()) {