LibWeb: Implement TextEncoderStream

Required by the server-side rendering mode of React Router, used by
https://chatgpt.com/

Note that the imported tests do not have the worker variants to prevent
freezing on macOS.

Libraries/LibWeb/Encoding/TextEncoderStream.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2025, Luke Wilde <luke@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/Bindings/PlatformObject.h>
+#include <LibWeb/Encoding/TextEncoderCommon.h>
+#include <LibWeb/Streams/GenericTransformStream.h>
+
+namespace Web::Encoding {
+
+class TextEncoderStream final
+    : public Bindings::PlatformObject
+    , public Streams::GenericTransformStreamMixin
+    , public TextEncoderCommonMixin {
+    WEB_PLATFORM_OBJECT(TextEncoderStream, Bindings::PlatformObject);
+    GC_DECLARE_ALLOCATOR(TextEncoderStream);
+
+public:
+    static WebIDL::ExceptionOr<GC::Ref<TextEncoderStream>> construct_impl(JS::Realm&);
+    virtual ~TextEncoderStream() override;
+
+private:
+    TextEncoderStream(JS::Realm&, GC::Ref<Streams::TransformStream>);
+
+    virtual void initialize(JS::Realm&) override;
+    virtual void visit_edges(Cell::Visitor&) override;
+
+    WebIDL::ExceptionOr<void> encode_and_enqueue_chunk(JS::Value);
+    WebIDL::ExceptionOr<void> encode_and_flush();
+
+    Optional<u32> convert_code_unit_to_scalar_value(u32 item, Utf8CodePointIterator& code_point_iterator);
+
+    // https://encoding.spec.whatwg.org/#textencoderstream-pending-high-surrogate
+    Optional<u32> m_leading_surrogate;
+};
+
+}