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/DevToolsServer.cpp

@@ -0,0 +1,107 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Debug.h>
+#include <AK/JsonObject.h>
+#include <AK/JsonValue.h>
+#include <LibCore/EventLoop.h>
+#include <LibCore/Socket.h>
+#include <LibCore/TCPServer.h>
+#include <LibDevTools/Actors/DeviceActor.h>
+#include <LibDevTools/Actors/PreferenceActor.h>
+#include <LibDevTools/Actors/ProcessActor.h>
+#include <LibDevTools/Connection.h>
+#include <LibDevTools/DevToolsServer.h>
+
+namespace DevTools {
+
+static u64 s_server_count = 0;
+
+ErrorOr<NonnullOwnPtr<DevToolsServer>> DevToolsServer::create(DevToolsDelegate& delegate, u16 port)
+{
+    auto address = IPv4Address::from_string("0.0.0.0"sv).release_value();
+
+    auto server = TRY(Core::TCPServer::try_create());
+    TRY(server->listen(address, port, Core::TCPServer::AllowAddressReuse::Yes));
+
+    return adopt_own(*new DevToolsServer(delegate, move(server)));
+}
+
+DevToolsServer::DevToolsServer(DevToolsDelegate& delegate, NonnullRefPtr<Core::TCPServer> server)
+    : m_server(move(server))
+    , m_delegate(delegate)
+    , m_server_id(s_server_count++)
+{
+    m_server->on_ready_to_accept = [this]() {
+        if (auto result = on_new_client(); result.is_error())
+            warnln("Failed to accept DevTools client: {}", result.error());
+    };
+}
+
+DevToolsServer::~DevToolsServer() = default;
+
+ErrorOr<void> DevToolsServer::on_new_client()
+{
+    if (m_connection)
+        return Error::from_string_literal("Only one active DevTools connection is currently allowed");
+
+    auto client = TRY(m_server->accept());
+    auto buffered_socket = TRY(Core::BufferedTCPSocket::create(move(client)));
+
+    m_connection = Connection::create(move(buffered_socket));
+
+    m_connection->on_connection_closed = [this]() {
+        close_connection();
+    };
+
+    m_connection->on_message_received = [this](auto const& message) {
+        on_message_received(message);
+    };
+
+    m_root_actor = register_actor<RootActor>();
+
+    register_actor<DeviceActor>();
+    register_actor<PreferenceActor>();
+    register_actor<ProcessActor>(ProcessDescription { .is_parent = true });
+
+    return {};
+}
+
+void DevToolsServer::on_message_received(JsonObject const& message)
+{
+    auto to = message.get_byte_string("to"sv);
+    if (!to.has_value()) {
+        m_root_actor->send_missing_parameter_error("to"sv);
+        return;
+    }
+
+    auto actor = m_actor_registry.find(*to);
+    if (actor == m_actor_registry.end()) {
+        m_root_actor->send_unknown_actor_error(*to);
+        return;
+    }
+
+    auto type = message.get_byte_string("type"sv);
+    if (!type.has_value()) {
+        actor->value->send_missing_parameter_error("type"sv);
+        return;
+    }
+
+    actor->value->handle_message(*type, message);
+}
+
+void DevToolsServer::close_connection()
+{
+    dbgln_if(DEVTOOLS_DEBUG, "Lost connection to the DevTools client");
+
+    Core::deferred_invoke([this]() {
+        m_connection = nullptr;
+        m_actor_registry.clear();
+        m_root_actor = nullptr;
+    });
+}
+
+}