stream: emit 'pause' on unpipe

unpipe should use pause() instead of mutating
state.flowing directly so that pausing side
effects such as emitting 'pause' are properly
performed.

Fixes: https://github.com/nodejs/node/issues/32470

PR-URL: https://github.com/nodejs/node/pull/32476


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>