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/ThreadActor.cpp

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibDevTools/Actors/ThreadActor.h>
+
+namespace DevTools {
+
+NonnullRefPtr<ThreadActor> ThreadActor::create(DevToolsServer& devtools, ByteString name)
+{
+    return adopt_ref(*new ThreadActor(devtools, move(name)));
+}
+
+ThreadActor::ThreadActor(DevToolsServer& devtools, ByteString name)
+    : Actor(devtools, move(name))
+{
+}
+
+ThreadActor::~ThreadActor() = default;
+
+void ThreadActor::handle_message(StringView type, JsonObject const&)
+{
+    send_unrecognized_packet_type_error(type);
+}
+
+}