AK+LibWeb: Move decode forgiving base64 under Web::Infra namespace

Since the forgiving base64 is part of the web infra standard

Userland/Libraries/LibWeb/Infra/Base64.h

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2022-2023, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+
+namespace Web::Infra {
+
+[[nodiscard]] ErrorOr<ByteBuffer> decode_forgiving_base64(StringView);
+
+}