LibWeb: Parse and resolve UnresolvedStyleValues

If a property is custom or contains a `var()` reference, it cannot be
parsed into a proper StyleValue immediately, so we store it as an
UnresolvedStyleValue until the property is compute. Then, at compute
time, we resolve them by expanding out any `var()` references, and
parsing the result.

The implementation here is very naive, and involves copying the
UnresolvedStyleValue's tree of StyleComponentValueRules while copying
the contents of any `var()`s it finds along the way. This is quite an
expensive operation to do every time that the style is computed.

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

@@ -113,6 +113,8 @@ public:
 
     RefPtr<StyleValue> parse_as_css_value(PropertyID);
 
+    static RefPtr<StyleValue> parse_css_value(Badge<StyleComputer>, ParsingContext const&, PropertyID, Vector<StyleComponentValueRule> const&);
+
 private:
     enum class ParsingResult {
         Done,