LibWeb: Move ReadableStream AOs into their own file

The main streams AO file has gotten very large, and is a bit difficult
to navigate. In an effort to improve DX, this migrates ReadableStream
AOs to their own file. And the helper classes used for the tee and pipe-
to operations are also in their own files.

Libraries/LibWeb/Streams/ReadableStreamPipeTo.h

@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Vector.h>
+#include <LibGC/Ptr.h>
+#include <LibJS/Forward.h>
+#include <LibJS/Heap/Cell.h>
+#include <LibWeb/DOM/AbortSignal.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::Streams::Detail {
+
+// https://streams.spec.whatwg.org/#ref-for-in-parallel
+class ReadableStreamPipeTo final : public JS::Cell {
+    GC_CELL(ReadableStreamPipeTo, JS::Cell);
+    GC_DECLARE_ALLOCATOR(ReadableStreamPipeTo);
+
+public:
+    void process();
+
+    void set_abort_signal(GC::Ref<DOM::AbortSignal>, DOM::AbortSignal::AbortSignal::AbortAlgorithmID);
+
+    void shutdown_with_action(GC::Ref<GC::Function<GC::Ref<WebIDL::Promise>()>> action, Optional<JS::Value> original_error = {});
+    void shutdown(Optional<JS::Value> error = {});
+
+private:
+    ReadableStreamPipeTo(
+        GC::Ref<JS::Realm>,
+        GC::Ref<WebIDL::Promise>,
+        GC::Ref<ReadableStream> source,
+        GC::Ref<WritableStream> destination,
+        GC::Ref<ReadableStreamDefaultReader> reader,
+        GC::Ref<WritableStreamDefaultWriter> writer,
+        bool prevent_close,
+        bool prevent_abort,
+        bool prevent_cancel);
+
+    virtual void visit_edges(Cell::Visitor& visitor) override;
+
+    void read_chunk();
+    void write_chunk();
+
+    void write_unwritten_chunks();
+    void wait_for_pending_writes_to_complete(Function<void()> on_complete);
+
+    void finish(Optional<JS::Value> error = {});
+
+    bool check_for_error_and_close_states();
+    bool check_for_forward_errors();
+    bool check_for_backward_errors();
+    bool check_for_forward_close();
+    bool check_for_backward_close();
+
+    GC::Ref<JS::Realm> m_realm;
+    GC::Ref<WebIDL::Promise> m_promise;
+
+    GC::Ref<ReadableStream> m_source;
+    GC::Ref<WritableStream> m_destination;
+
+    GC::Ref<ReadableStreamDefaultReader> m_reader;
+    GC::Ref<WritableStreamDefaultWriter> m_writer;
+
+    GC::Ptr<DOM::AbortSignal> m_signal;
+    DOM::AbortSignal::AbortAlgorithmID m_signal_id { 0 };
+
+    Vector<GC::Ref<WebIDL::Promise>> m_pending_writes;
+    Vector<JS::Value, 1> m_unwritten_chunks;
+
+    bool m_prevent_close { false };
+    bool m_prevent_abort { false };
+    bool m_prevent_cancel { false };
+
+    bool m_shutting_down { false };
+};
+
+}