LibDevTools: Implement enough of the protocol to inspect tabs

There is a lot needed all at once to actually inspect a tab's DOM tree.
It begins with requesting a "watcher" from a TabActor. It seems there
can be many types of watchers, but here we implement the "frame" watcher
only. The watcher creates an "inspector", which in turn creates a
"walker", which is the actor ultimately responsible for serializing and
inspecting the DOM tree.

In between all that, the DevTools client will send a handful of other
informational requests. If we do not reply to these, the client will not
move forward with the walker. For example, the CSSPropertiesActor will
be asked for a list of all known CSS properties.

Libraries/LibDevTools/Actors/CSSPropertiesActor.cpp

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/JsonArray.h>
+#include <AK/JsonObject.h>
+#include <LibDevTools/Actors/CSSPropertiesActor.h>
+#include <LibDevTools/DevToolsDelegate.h>
+#include <LibDevTools/DevToolsServer.h>
+
+namespace DevTools {
+
+NonnullRefPtr<CSSPropertiesActor> CSSPropertiesActor::create(DevToolsServer& devtools, ByteString name)
+{
+    return adopt_ref(*new CSSPropertiesActor(devtools, move(name)));
+}
+
+CSSPropertiesActor::CSSPropertiesActor(DevToolsServer& devtools, ByteString name)
+    : Actor(devtools, move(name))
+{
+}
+
+CSSPropertiesActor::~CSSPropertiesActor() = default;
+
+void CSSPropertiesActor::handle_message(StringView type, JsonObject const&)
+{
+    JsonObject response;
+    response.set("from"sv, name());
+
+    if (type == "getCSSDatabase"sv) {
+        auto css_property_list = devtools().delegate().css_property_list();
+
+        JsonObject properties;
+
+        for (auto const& css_property : css_property_list) {
+            JsonArray subproperties;
+            subproperties.must_append(css_property.name);
+
+            JsonObject property;
+            property.set("isInherited"sv, css_property.is_inherited);
+            property.set("supports"sv, JsonArray {});
+            property.set("values"sv, JsonArray {});
+            property.set("subproperties"sv, move(subproperties));
+
+            properties.set(css_property.name, move(property));
+        }
+
+        response.set("properties"sv, move(properties));
+        send_message(move(response));
+        return;
+    }
+
+    send_unrecognized_packet_type_error(type);
+}
+
+}