LibWeb: Generate shorthand initial values after their longhands

When parsing shorthand values, we'd like to use
`property_initial_value()` to get their longhand property values,
instead of hard-coding them as we currently do. That involves
recursively calling that function while the `initial_values` map is
being initialized, which causes problems because the shorthands appear
alphabetically before their longhand components, so the longhands aren't
initialized yet!

The solution here is to perform 2 passes when generating the code,
outputting properties without "longhands" first, and the rest after.
This could potentially cause issues when shorthands have multiple
levels, in particular `border` -> `border-color` -> `border-left-color`.
But, we do not currently define a default value for `border`, and
`border-color` takes only a single value, so it's fine for now. :^)

Meta/Lagom/Tools/CodeGenerators/LibWeb/Generate_CSS_PropertyID_cpp.cpp

@@ -164,11 +164,14 @@ RefPtr<StyleValue> property_initial_value(PropertyID property_id)
         ParsingContext parsing_context;
 )~~~");
 
-    properties.for_each_member([&](auto& name, auto& value) {
+    // NOTE: Parsing a shorthand property requires that its longhands are already available here.
-        VERIFY(value.is_object());
+    //       So, we do this in two passes: First longhands, then shorthands.
+    //       Probably we should build a dependency graph and then handle them in order, but this
+    //       works for now! :^)
 
-        if (value.as_object().has("initial")) {
+    auto output_initial_value_code = [&](auto& name, auto& object) {
-            auto& initial_value = value.as_object().get("initial");
+        if (object.has("initial")) {
+            auto& initial_value = object.get("initial");
             VERIFY(initial_value.is_string());
             auto initial_value_string = initial_value.as_string();
 
@@ -180,11 +183,31 @@ RefPtr<StyleValue> property_initial_value(PropertyID property_id)
             initial_values.set(PropertyID::@name:titlecase@, parsed_value.release_nonnull());
 )~~~");
         }
+    };
+
+    properties.for_each_member([&](auto& name, auto& value) {
+        VERIFY(value.is_object());
+        if (value.as_object().has("longhands"))
+            return;
+        output_initial_value_code(name, value.as_object());
+    });
+
+    properties.for_each_member([&](auto& name, auto& value) {
+        VERIFY(value.is_object());
+        if (!value.as_object().has("longhands"))
+            return;
+        output_initial_value_code(name, value.as_object());
     });
 
     generator.append(R"~~~(
     }
-    return initial_values.get(property_id).value_or(nullptr);
+
+    auto maybe_value = initial_values.get(property_id);
+    if (!maybe_value.has_value()) {
+        dbgln("No initial value found for '{}' property!", string_from_property_id(property_id));
+        return nullptr;
+    }
+    return maybe_value.value();
 }
 
 bool property_has_quirk(PropertyID property_id, Quirk quirk)