LibJS+LibWeb: Implement resizable ArrayBuffer support for DataView

This is (part of) a normative change in the ECMA-262 spec. See:
a9ae96e

Userland/Libraries/LibJS/Runtime/ByteLength.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+#include <AK/Variant.h>
+
+namespace JS {
+
+class ByteLength {
+public:
+    static ByteLength auto_() { return { Auto {} }; }
+    static ByteLength detached() { return { Detached {} }; }
+
+    ByteLength(u32 length)
+        : m_length(length)
+    {
+    }
+
+    bool is_auto() const { return m_length.has<Auto>(); }
+    bool is_detached() const { return m_length.has<Detached>(); }
+
+    u32 length() const
+    {
+        VERIFY(m_length.has<u32>());
+        return m_length.get<u32>();
+    }
+
+private:
+    struct Auto { };
+    struct Detached { };
+
+    using Length = Variant<Auto, Detached, u32>;
+
+    ByteLength(Length length)
+        : m_length(move(length))
+    {
+    }
+
+    Length m_length;
+};
+
+}