Take n-api out of experimental as per:
nodejs/TSC#501

PR-URL: #19262
Reviewed-By: Matteo Collina [email protected]
Reviewed-By: Colin Ihrig [email protected]
Reviewed-By: Michaël Zasso [email protected]
Reviewed-By: Gus Caplan [email protected]
Reviewed-By: James M Snell [email protected]
Reviewed-By: Franziska Hinkelmann [email protected]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines