path: fix regression in posix.normalize

Fixes a regression introduced in [1].
The posix version of normalize should not treat backslash as a path
separator.

[1] https://github.com/nodejs/node/commit/4ae320f2

PR-URL: https://github.com/nodejs/node/pull/19520
Fixes: https://github.com/nodejs/node/issues/19519


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>