Add NEXT_MANUAL_SIG_HANDLE handling to start-server.ts (#59117)
If a manual signal handler is registered, SIGINT and SIGTERM should not
be handled by Next.js. This was already the case in the standalone
server.js but was missing here, rendering the env flag useless.
With this fix, the example given in
https://nextjs.org/docs/pages/building-your-application/deploying#manual-graceful-shutdowns
is working (again).
Fixes #56810.
Co-authored-by: Zack Tanner <zacktanner@gmail.com>
