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/Actors/RootActor.cpp

@@ -0,0 +1,136 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/JsonObject.h>
+#include <LibDevTools/Actors/DeviceActor.h>
+#include <LibDevTools/Actors/PreferenceActor.h>
+#include <LibDevTools/Actors/ProcessActor.h>
+#include <LibDevTools/Actors/RootActor.h>
+#include <LibDevTools/DevToolsDelegate.h>
+#include <LibDevTools/DevToolsServer.h>
+
+namespace DevTools {
+
+// https://firefox-source-docs.mozilla.org/devtools/backend/protocol.html#the-root-actor
+NonnullRefPtr<RootActor> RootActor::create(DevToolsServer& devtools, ByteString name)
+{
+    auto actor = adopt_ref(*new RootActor(devtools, move(name)));
+
+    JsonObject traits;
+    traits.set("sources"sv, false);
+    traits.set("highlightable"sv, true);
+    traits.set("customHighlighters"sv, true);
+    traits.set("networkMonitor"sv, false);
+
+    JsonObject message;
+    message.set("from"sv, actor->name());
+    message.set("applicationType"sv, "browser"sv);
+    message.set("traits"sv, move(traits));
+    actor->send_message(move(message));
+
+    return actor;
+}
+
+RootActor::RootActor(DevToolsServer& devtools, ByteString name)
+    : Actor(devtools, move(name))
+{
+}
+
+RootActor::~RootActor() = default;
+
+void RootActor::handle_message(StringView type, JsonObject const& message)
+{
+    JsonObject response;
+    response.set("from"sv, name());
+
+    if (type == "connect") {
+        send_message(move(response));
+        return;
+    }
+
+    if (type == "getRoot"sv) {
+        response.set("selected"sv, 0);
+
+        for (auto const& actor : devtools().actor_registry()) {
+            if (is<DeviceActor>(*actor.value))
+                response.set("deviceActor"sv, actor.key);
+            else if (is<PreferenceActor>(*actor.value))
+                response.set("preferenceActor"sv, actor.key);
+        }
+
+        send_message(move(response));
+        return;
+    }
+
+    if (type == "getProcess"sv) {
+        auto id = message.get_integer<u64>("id"sv);
+        if (!id.has_value()) {
+            send_missing_parameter_error("id"sv);
+            return;
+        }
+
+        for (auto const& actor : devtools().actor_registry()) {
+            auto const* process_actor = as_if<ProcessActor>(*actor.value);
+            if (!process_actor)
+                continue;
+            if (process_actor->description().id != *id)
+                continue;
+
+            response.set("processDescriptor"sv, process_actor->serialize_description());
+            break;
+        }
+
+        send_message(move(response));
+        return;
+    }
+
+    if (type == "getTab"sv) {
+        response.set("tab"sv, JsonObject {});
+        send_message(move(response));
+        return;
+    }
+
+    if (type == "listAddons"sv) {
+        response.set("addons"sv, JsonArray {});
+        send_message(move(response));
+        return;
+    }
+
+    if (type == "listProcesses"sv) {
+        JsonArray processes;
+
+        for (auto const& actor : devtools().actor_registry()) {
+            if (auto const* process_actor = as_if<ProcessActor>(*actor.value))
+                processes.must_append(process_actor->serialize_description());
+        }
+
+        response.set("processes"sv, move(processes));
+        send_message(move(response));
+        return;
+    }
+
+    if (type == "listServiceWorkerRegistrations"sv) {
+        response.set("registrations"sv, JsonArray {});
+        send_message(move(response));
+        return;
+    }
+
+    if (type == "listTabs"sv) {
+        response.set("tabs"sv, JsonArray {});
+        send_message(move(response));
+        return;
+    }
+
+    if (type == "listWorkers"sv) {
+        response.set("workers"sv, JsonArray {});
+        send_message(move(response));
+        return;
+    }
+
+    send_unrecognized_packet_type_error(type);
+}
+
+}