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/WatcherActor.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <LibDevTools/Actor.h>
+
+namespace DevTools {
+
+class WatcherActor final : public Actor {
+public:
+    static constexpr auto base_name = "watcher"sv;
+
+    static NonnullRefPtr<WatcherActor> create(DevToolsServer&, ByteString name, WeakPtr<TabActor>);
+    virtual ~WatcherActor() override;
+
+    virtual void handle_message(StringView type, JsonObject const&) override;
+
+    JsonObject serialize_description() const;
+
+private:
+    WatcherActor(DevToolsServer&, ByteString name, WeakPtr<TabActor>);
+
+    WeakPtr<TabActor> m_tab;
+    WeakPtr<Actor> m_target;
+    WeakPtr<TargetConfigurationActor> m_target_configuration;
+    WeakPtr<ThreadConfigurationActor> m_thread_configuration;
+};
+
+}