LibWeb/CSS: Add tokenize() method to style values, to get a list of CVs

A couple of arbitrary substitution functions require us to get or
produce some style value, and then substitute its ComponentValues into
the original ComponentValue list. So this commit gives CSSStyleValue a
tokenize() method that does so.

Apart from a couple of unusual cases like the guaranteed-invalid value,
style values can all be converted into ComponentValues by serializing
them as a string, and then parsing that as a list of component values.
That feels unnecessarily inefficient in most cases though, so I've
implemented faster overrides for a lot of the basic style value
classes, but left that serialize-and-reparse method as the fallback.

Libraries/LibWeb/CSS/StyleValues/UnresolvedStyleValue.h

@@ -24,6 +24,7 @@ public:
     virtual ~UnresolvedStyleValue() override = default;
 
     virtual String to_string(SerializationMode) const override;
+    virtual Vector<Parser::ComponentValue> tokenize() const override { return m_values; }
 
     Vector<Parser::ComponentValue> const& values() const { return m_values; }
     bool contains_arbitrary_substitution_function() const { return m_contains_arbitrary_substitution_function; }