LibIMAP: Add quoted printable decoder

This is a very common encoding for e-mail. Gmail seems to encode all
HTML e-mail in it.
imap qp clang

Userland/Libraries/LibIMAP/QuotedPrintable.h

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/ByteBuffer.h>
+
+namespace IMAP {
+
+ByteBuffer decode_quoted_printable(StringView const&);
+
+}