workers: fix spawning from preload scripts

Fix spawning nested worker threads from preload scripts and
warn about doing so.

Signed-off-by: James M Snell <jasnell@gmail.com>
Fixes: https://github.com/nodejs/node/issues/36531

PR-URL: https://github.com/nodejs/node/pull/37481


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