src: fix return type of method in string_search.h

`Vector::forward()` is supposed to return a `bool`.

PR-URL: https://github.com/nodejs/node/pull/37167


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>