LibWeb: Generate a ValueListStyleValue when parsing CSS values

We skip whitespace tokens while doing this. As far as I can tell,
whitespace is not useful once we get to this point, and it legally
may or may not appear between any two tokens. By not including it
in the ValueListStyleValue, we make the "if it has 3 parts"-type
checks a lot more straightforward.

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

@@ -177,6 +177,8 @@ private:
 
     static Optional<float> try_parse_float(StringView string);
 
+    RefPtr<StyleValue> parse_single_css_value(PropertyID, StyleComponentValueRule const&);
+
     ParsingContext m_context;
 
     Tokenizer m_tokenizer;