add Statsig Node Core package to Server External Packages (#84157)
### What?
Statsig is a popular AB Testing/ Feature Flagging solution to use in concert with Next.js. Today, their newest Server SDK doesn't play nicely with Next as its build around a Rust library, which doesn't play nice with webpack and causes build issues.
### Why?
This'll remove a step for Statsig/Next developers having to troubleshoot and add @statsig/statsig-node-core to serverExternalPackages themselves.
### How?
Added to the list published here: https://nextjs.org/docs/app/api-reference/config/next-config-js/serverExternalPackages