feat(nextjs): Emit warning if your app directory doesn't have a global-error.js file

[lforst]

We should warn users if no global-error.js file is present in the app directory because that essentially means there is an instrumentation gap in their setup.

https://nextjs.org/docs/app/building-your-application/routing/error-handling#handling-errors-in-root-layouts

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	66 KB (+0.21% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	59.6 KB (+0.23% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.12 KB (+0.02% 🔺)
@sentry/browser - Webpack (gzipped)	21.38 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	62.96 KB (+0.4% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	29.64 KB (+0.39% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	21.7 KB (+0.49% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	198.32 KB (+0.44% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	89.75 KB (+0.48% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	64.73 KB (+0.68% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	32.3 KB (+0.26% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	66.36 KB (+0.17% 🔺)
@sentry/react - Webpack (gzipped)	21.43 KB (+0.04% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	83.08 KB (+0.14% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	48.22 KB (+0.02% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	16.19 KB (-0.03% 🔽)
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	74.65 KB (added)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	29.73 KB (added)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	71.24 KB (added)