string_decoder: reset decoder on end

This resets the StringDecoder's state after calling `#end`. Further
writes to the decoder will act as if it were a brand new instance,
allowing simple reuse.

PR-URL: https://github.com/nodejs/node/pull/18494
Fixes: https://github.com/nodejs/node/issues/16564
Refs: https://github.com/nodejs/node/pull/16594


Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>