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/StyleRules.cpp

@@ -57,7 +57,13 @@ StyleDeclarationRule::StyleDeclarationRule() { }
 StyleDeclarationRule::~StyleDeclarationRule() { }
 
 StyleFunctionRule::StyleFunctionRule(String name)
-    : m_name(name)
+    : m_name(move(name))
+{
+}
+
+StyleFunctionRule::StyleFunctionRule(String name, Vector<StyleComponentValueRule>&& values)
+    : m_name(move(name))
+    , m_values(move(values))
 {
 }
 StyleFunctionRule::~StyleFunctionRule() { }