LibWeb: Break friendship between CSS Declaration and Parser

This means deviating slightly from the spec in order to construct a
fully-initialized Declaration instead of creating an empty one and then
poking at its internals.

DeclarationOrAtRule should probably use a Variant, but for now, making
its Declaration member optional is quick and easy.

Userland/Libraries/LibWeb/CSS/Parser/Declaration.cpp

@@ -10,7 +10,13 @@
 
 namespace Web::CSS::Parser {
 
-Declaration::Declaration() = default;
+Declaration::Declaration(FlyString name, Vector<ComponentValue> values, Important important)
+    : m_name(move(name))
+    , m_values(move(values))
+    , m_important(move(important))
+{
+}
+
 Declaration::~Declaration() = default;
 
 String Declaration::to_string() const