Add "@zenstackhq/runtime" to server-external-packages.json (#54829)
### What?
[ZenStack](https://github.com/zenstackhq/zenstack) is a toolkit built
above Prisma ORM for adding access control and other features. This
change adds one of its packages "@zenstackhq/runtime" to the default
list of "serverComponentsExternalPackages".
### Why?
The package is frequently used in Next.js route handlers and results in
false-positive errors with Next.js's bundler (likely caused by
dynamically `require` modules).
### How?
Add the package to the default external packages list.
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Co-authored-by: Tim Neutkens <tim@timneutkens.nl>