LibWeb: Re-implement HTML::Navigator using IDL

Get rid of the bespoke NavigatorObject class and use the modern IDL
strategies for creating platform objects to re-implement Navigator and
its associcated mixin interfaces. While we're here, implement it in a
way that brings WorkerNavigator up to spec :^)

Userland/Libraries/LibWeb/HTML/NavigatorOnLine.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2022, Andrew Kaster <akaster@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Web::HTML {
+
+class NavigatorOnLineMixin {
+public:
+    // https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-online
+    // FIXME: Reflect actual connectivity status.
+    bool on_line() const { return true; }
+};
+
+}