feat(nextjs): Add experimental_captureRequestError for onRequestError hook

[lforst]

This PR adds an experimental implementation of an error handler that can be used to capture errors for the onRequestError hook recently added to Next.js canary: vercel/next.js#67539

The handler can be added to the instrumentation.ts file as follows:

// instrumentation.ts
import * as Sentry from '@sentry/nextjs';

export async function register() {
  if (process.env.NEXT_RUNTIME === 'nodejs') {
    await import('./sentry.server.config');
  }

  if (process.env.NEXT_RUNTIME === 'edge') {
    await import('./sentry.edge.config');
  }
}

export const onRequestError = Sentry.experimental_captureRequestError;

Currently, Next.js requires a __NEXT_EXPERIMENTAL_INSTRUMENTATION=1 environment variable for this feature to work.

[chargome]

Nice!