LibWeb: Ignore vendor-prefixed pseudo-element/classes in selectors

Our debug logging when we fail to parse a legitimate selector, is less
useful when it's hidden among selectors that don't parse because they
contain vendor-prefixed pseudo-elements and classes. So, now we
specifically ignore these and don't produce a log message.

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

@@ -102,6 +102,7 @@ public:
 private:
     enum class SelectorParsingResult {
         Done,
+        IncludesIgnoredVendorPrefix,
         SyntaxError,
     };