LibWeb/CSS: Use SelectorList type instead of Vector<NNRP<Selector>>

Author: https://github.com/AtkinsSJ
Commit: 74c448d744
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1842
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/CSS/Selector.cpp

@@ -407,7 +407,7 @@ String Selector::serialize() const
 }
 
 // https://www.w3.org/TR/cssom/#serialize-a-group-of-selectors
-String serialize_a_group_of_selectors(Vector<NonnullRefPtr<Selector>> const& selectors)
+String serialize_a_group_of_selectors(SelectorList const& selectors)
 {
     // To serialize a group of selectors serialize each selector in the group of selectors and then serialize a comma-separated list of these serializations.
     return MUST(String::join(", "sv, selectors));