LibWeb: Begin implementing SharedWorker

Shared workers are essentially just workers that may be accessed from
scripts within the same origin. There are plenty of FIXMEs here (mostly
building on existing worker FIXMEs that are already in place), but this
lets us run the shared worker variants of WPT tests.

Libraries/LibWeb/HTML/SharedWorker.h

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibURL/URL.h>
+#include <LibWeb/Forward.h>
+#include <LibWeb/HTML/AbstractWorker.h>
+#include <LibWeb/HTML/WorkerAgentParent.h>
+#include <LibWeb/WebIDL/ExceptionOr.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/workers.html#dedicated-workers-and-the-worker-interface
+class SharedWorker final
+    : public DOM::EventTarget
+    , public HTML::AbstractWorker {
+    WEB_PLATFORM_OBJECT(SharedWorker, DOM::EventTarget);
+    GC_DECLARE_ALLOCATOR(SharedWorker);
+
+public:
+    static WebIDL::ExceptionOr<GC::Ref<SharedWorker>> construct_impl(JS::Realm&, String const& script_url, Variant<String, WorkerOptions>& options);
+
+    virtual ~SharedWorker();
+
+    GC::Ref<MessagePort> port() { return m_port; }
+
+    void set_agent(WorkerAgentParent& agent) { m_agent = agent; }
+
+private:
+    SharedWorker(JS::Realm&, URL::URL script_url, WorkerOptions, MessagePort&);
+
+    // ^AbstractWorker
+    virtual DOM::EventTarget& this_event_target() override { return *this; }
+
+    virtual void initialize(JS::Realm&) override;
+    virtual void visit_edges(Cell::Visitor&) override;
+
+    URL::URL m_script_url;
+    WorkerOptions m_options;
+    GC::Ref<MessagePort> m_port;
+    GC::Ptr<WorkerAgentParent> m_agent;
+};
+
+}