LibWeb: Add 'byte-{lower,upper}case' operations from the Infra spec

Usually operations that mirror AOs from the Infra spec are simply part
of the underlying data structures in AK directly, but these don't seem
generally useful enough to add them as ByteBuffer methods.

Userland/Libraries/LibWeb/Infra/ByteSequences.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/ByteBuffer.h>
+
+namespace Web::Infra {
+
+void byte_lowercase(ByteBuffer&);
+void byte_uppercase(ByteBuffer&);
+
+}