LibWeb: Add Transformer API

This adds the Transformer API which the TransformStream() constructor
accepts as its first argument.

Userland/Libraries/LibWeb/Streams/Transformer.cpp

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2023, Kenneth Myhra <kennethmyhra@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/VM.h>
+#include <LibWeb/Streams/AbstractOperations.h>
+#include <LibWeb/Streams/Transformer.h>
+#include <LibWeb/WebIDL/CallbackType.h>
+
+namespace Web::Streams {
+
+JS::ThrowCompletionOr<Transformer> Transformer::from_value(JS::VM& vm, JS::Value value)
+{
+    if (!value.is_object())
+        return Transformer {};
+
+    auto& object = value.as_object();
+
+    Transformer transformer {
+        .start = TRY(property_to_callback(vm, value, "start", WebIDL::OperationReturnsPromise::No)),
+        .transform = TRY(property_to_callback(vm, value, "transform", WebIDL::OperationReturnsPromise::Yes)),
+        .flush = TRY(property_to_callback(vm, value, "flush", WebIDL::OperationReturnsPromise::Yes)),
+        .readable_type = {},
+        .writable_type = {},
+    };
+
+    if (TRY(object.has_property("readableType")))
+        transformer.readable_type = TRY(object.get("readableType"));
+
+    if (TRY(object.has_property("writableType")))
+        transformer.writable_type = TRY(object.get("writableType"));
+
+    return transformer;
+}
+
+}

Userland/Libraries/LibWeb/Streams/Transformer.h

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+/*
+ * Copyright (c) 2023, Kenneth Myhra <kennethmyhra@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Forward.h>
+#include <LibJS/Heap/Handle.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::Streams {
+
+// https://streams.spec.whatwg.org/#dictdef-transformer
+struct Transformer {
+    // https://streams.spec.whatwg.org/#dom-transformer-start
+    JS::Handle<WebIDL::CallbackType> start;
+    // https://streams.spec.whatwg.org/#dom-transformer-transform
+    JS::Handle<WebIDL::CallbackType> transform;
+    // https://streams.spec.whatwg.org/#dom-transformer-flush
+    JS::Handle<WebIDL::CallbackType> flush;
+
+    // https://streams.spec.whatwg.org/#dom-transformer-readabletype
+    Optional<JS::Value> readable_type;
+    // https://streams.spec.whatwg.org/#dom-transformer-writabletype
+    Optional<JS::Value> writable_type;
+
+    static JS::ThrowCompletionOr<Transformer> from_value(JS::VM&, JS::Value);
+};
+
+}