fix(nextjs): Use globalThis for code injection

[timfish]

Since v8, all platforms and versions we support have support for globalThis. Since this PR we also use globalThis when reading these injectected values.

[Lms24]

Nice, thx!

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser	22.45 KB (0%)
@sentry/browser (incl. Tracing)	34.22 KB (0%)
@sentry/browser (incl. Tracing, Replay)	70.28 KB (0%)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	63.62 KB (0%)
@sentry/browser (incl. Tracing, Replay with Canvas)	74.68 KB (0%)
@sentry/browser (incl. Tracing, Replay, Feedback)	87.26 KB (0%)
@sentry/browser (incl. Tracing, Replay, Feedback, metrics)	89.11 KB (0%)
@sentry/browser (incl. metrics)	26.75 KB (0%)
@sentry/browser (incl. Feedback)	39.37 KB (0%)
@sentry/browser (incl. sendFeedback)	27.06 KB (0%)
@sentry/browser (incl. FeedbackAsync)	31.7 KB (0%)
@sentry/react	25.22 KB (0%)
@sentry/react (incl. Tracing)	37.22 KB (0%)
@sentry/vue	26.6 KB (0%)
@sentry/vue (incl. Tracing)	36.06 KB (0%)
@sentry/svelte	22.58 KB (0%)
CDN Bundle	23.64 KB (0%)
CDN Bundle (incl. Tracing)	35.88 KB (0%)
CDN Bundle (incl. Tracing, Replay)	70.31 KB (0%)
CDN Bundle (incl. Tracing, Replay, Feedback)	75.57 KB (0%)
CDN Bundle - uncompressed	69.37 KB (0%)
CDN Bundle (incl. Tracing) - uncompressed	106.31 KB (0%)
CDN Bundle (incl. Tracing, Replay) - uncompressed	218.16 KB (0%)
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	230.99 KB (0%)
@sentry/nextjs (client)	37.07 KB (-0.04% 🔽)
@sentry/sveltekit (client)	34.79 KB (-0.06% 🔽)
@sentry/node	114.55 KB (0%)
@sentry/node - without tracing	89.33 KB (0%)
@sentry/aws-serverless	98.5 KB (0%)