string_decoder: fix regressions

There are libraries which invoke StringDecoder using .call and
.inherits, which directly conflicts with making StringDecoder
be a class which can only be invoked with the new keyword.
Revert to declaring it as a function.

StringDecoder#lastNeed was not defined, redefine it using
the new interface and fix StringDecoder#lastTotal.

PR-URL: https://github.com/nodejs/node/pull/18723
Refs: https://github.com/nodejs/node/pull/18537


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>