src: allow UTF-16 in generic StringBytes decode call

This allows removing a number of special cases in other
parts of the code, at least one of which was incorrect
in expecting aligned input when that was not guaranteed.

Fixes: https://github.com/nodejs/node/issues/22358

PR-URL: https://github.com/nodejs/node/pull/22622


Reviewed-By: Tobias Nießen <tniessen@tnie.de>