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/StyleFunctionRule.h

@@ -20,7 +20,8 @@ class StyleFunctionRule : public RefCounted<StyleFunctionRule> {
     friend class Parser;
 
 public:
-    StyleFunctionRule(String name);
+    explicit StyleFunctionRule(String name);
+    StyleFunctionRule(String name, Vector<StyleComponentValueRule>&& values);
     ~StyleFunctionRule();
 
     String const& name() const { return m_name; }