LibWeb: Add TransformStreamDefaultController scaffolding

This adds the scaffolding of TransformStreamDefaultController so we can
start implementing the necessary abstract operations for it.

Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2023, Kenneth Myhra <kennethmyhra@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/Bindings/PlatformObject.h>
+#include <LibWeb/Streams/AbstractOperations.h>
+
+namespace Web::Streams {
+
+class TransformStreamDefaultController : public Bindings::PlatformObject {
+    WEB_PLATFORM_OBJECT(TransformStreamDefaultController, Bindings::PlatformObject);
+
+public:
+    explicit TransformStreamDefaultController(JS::Realm&);
+    virtual ~TransformStreamDefaultController() override;
+
+    Optional<double> desired_size();
+
+    auto& flush_algorithm() { return m_flush_algorithm; }
+    void set_flush_algorithm(Optional<FlushAlgorithm>&& value) { m_flush_algorithm = move(value); }
+
+    auto& transform_algorithm() { return m_transform_algorithm; }
+    void set_transform_algorithm(Optional<TransformAlgorithm>&& value) { m_transform_algorithm = move(value); }
+
+    JS::GCPtr<TransformStream> stream() { return m_stream; }
+    void set_stream(JS::GCPtr<TransformStream> stream) { m_stream = stream; }
+
+private:
+    virtual JS::ThrowCompletionOr<void> initialize(JS::Realm&) override;
+
+    virtual void visit_edges(Cell::Visitor&) override;
+
+    // https://streams.spec.whatwg.org/#transformstreamdefaultcontroller-flushalgorithm
+    Optional<FlushAlgorithm> m_flush_algorithm;
+
+    // https://streams.spec.whatwg.org/#transformstreamdefaultcontroller-transformalgorithm
+    Optional<TransformAlgorithm> m_transform_algorithm;
+
+    // https://streams.spec.whatwg.org/#transformstreamdefaultcontroller-stream
+    JS::GCPtr<TransformStream> m_stream;
+};
+
+}