LibDevTools: Introduce a Firefox DevTools server library

To aid with debugging web page issues in Ladybird without needing to
implement a fully fledged inspector, we can implement the Firefox
DevTools protocol and use their DevTools. The protocol is described
here:

https://firefox-source-docs.mozilla.org/devtools/backend/protocol.html

This commit contains just enough to connect to Ladybird from a DevTools
client.

Libraries/LibDevTools/Actor.h

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Badge.h>
+#include <AK/ByteString.h>
+#include <AK/Optional.h>
+#include <AK/RefCounted.h>
+#include <AK/StringView.h>
+#include <AK/Vector.h>
+#include <AK/WeakPtr.h>
+#include <AK/Weakable.h>
+#include <LibDevTools/Forward.h>
+
+namespace DevTools {
+
+class Actor
+    : public RefCounted<Actor>
+    , public Weakable<Actor> {
+public:
+    virtual ~Actor();
+
+    ByteString const& name() const { return m_name; }
+    virtual void handle_message(StringView type, JsonObject const&) = 0;
+
+    class [[nodiscard]] BlockToken {
+    public:
+        BlockToken(Badge<Actor>, Actor&);
+        ~BlockToken();
+
+        BlockToken(BlockToken const&) = delete;
+        BlockToken& operator=(BlockToken const&) = delete;
+
+        BlockToken(BlockToken&&);
+        BlockToken& operator=(BlockToken&&);
+
+    private:
+        WeakPtr<Actor> m_actor;
+    };
+
+    void send_message(JsonValue, Optional<BlockToken> block_token = {});
+    void send_missing_parameter_error(StringView parameter);
+    void send_unrecognized_packet_type_error(StringView type);
+    void send_unknown_actor_error(StringView actor);
+
+protected:
+    explicit Actor(DevToolsServer&, ByteString name);
+
+    DevToolsServer& devtools() { return m_devtools; }
+    DevToolsServer const& devtools() const { return m_devtools; }
+
+    BlockToken block_responses();
+
+private:
+    DevToolsServer& m_devtools;
+    ByteString m_name;
+
+    Vector<JsonValue> m_blocked_responses;
+    bool m_block_responses { false };
+};
+
+}