LibIMAP+Mail: Implement RFC2047 message header encoding

This enables us to display email subject fields with non-ASCII
characters in Mail :^)

Userland/Libraries/LibIMAP/MessageHeaderEncoding.h

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2023, Valtteri Koskivuori <vkoskiv@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/ByteBuffer.h>
+
+namespace IMAP {
+
+ErrorOr<ByteBuffer> decode_rfc2047_encoded_words(StringView input);
+
+}