string_decoder: reimplement in C++

Implement string decoder in C++. The perks are a decent speed boost
(for decoding, whereas creation show some performance degradation),
that this can now be used more easily to add native decoding support
to C++ streams and (arguably) more readable variable names.

PR-URL: https://github.com/nodejs/node/pull/18537


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>