LibWeb: Prevent copying CSS TokenStream

This was leading to confusing bugs where I was accidentally passing it
by value and then wondering why tokens weren't getting consumed!

Userland/Libraries/LibWeb/CSS/Parser/Parser.h

@@ -57,6 +57,8 @@ public:
     explicit TokenStream(Vector<T> const&);
     ~TokenStream();
 
+    TokenStream(TokenStream<T> const&) = delete;
+
     bool has_next_token();
     T const& next_token();
     T const& peek_token(int offset = 0);