Add warn and telemetry for customized esmExternals (#67339)
### What
* Warn with next.js when users customized `experimental.esmExternals`
value
* Add telemetry tracking on the customization usage for that flag. 0 for
no customization, 1 for used non-default customized value
### Why
`esmExternals` ideally can just remain as default value `true` which
Next.js can handle the customization properly. Since next.js app router
also supports it on canary now we're adding a warning to users that
don't modify `esmExternals` option as it could affect module resolution
on ESM packages.
