ref(nextjs): Improve app router routing instrumentation accuracy

[lforst]

Improves the Next.js routing instrumentation by patching the Next.js router and instrumenting window popstates.

A few details on this PR that might explain weird-looking logic:

• The patching of the router is in a setInterval because Next.js may take a while to write the router to the window object and we don't have a cue when that has happened.
• We are using a combination of patching router.back/router.forward and the popstate event to emit a properly named transaction, because router.back and router.forward aren't passed any useful strings we could use as txn names.
• Because there is a slight delay between the router.back/router.forward calls and the popstate event, we temporarily give the navigation span an invalid name that we use as an indicator to drop if one may leak through.

[github-actions]

size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	22.52 KB	-	-
@sentry/browser - with treeshaking flags	21.3 KB	-	-
@sentry/browser (incl. Tracing)	34.8 KB	-	-
@sentry/browser (incl. Tracing, Replay)	71.26 KB	-	-
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	61.7 KB	-	-
@sentry/browser (incl. Tracing, Replay with Canvas)	75.61 KB	-	-
@sentry/browser (incl. Tracing, Replay, Feedback)	88.39 KB	-	-
@sentry/browser (incl. Tracing, Replay, Feedback, metrics)	90.23 KB	-	-
@sentry/browser (incl. metrics)	26.83 KB	-	-
@sentry/browser (incl. Feedback)	39.66 KB	-	-
@sentry/browser (incl. sendFeedback)	27.19 KB	-	-
@sentry/browser (incl. FeedbackAsync)	31.96 KB	-	-
@sentry/react	25.28 KB	-	-
@sentry/react (incl. Tracing)	37.77 KB	-	-
@sentry/vue	26.72 KB	-	-
@sentry/vue (incl. Tracing)	36.67 KB	-	-
@sentry/svelte	22.66 KB	-	-
CDN Bundle	23.83 KB	-	-
CDN Bundle (incl. Tracing)	36.56 KB	-	-
CDN Bundle (incl. Tracing, Replay)	71.02 KB	-	-
CDN Bundle (incl. Tracing, Replay, Feedback)	76.33 KB	-	-
CDN Bundle - uncompressed	69.81 KB	-	-
CDN Bundle (incl. Tracing) - uncompressed	108.44 KB	-	-
CDN Bundle (incl. Tracing, Replay) - uncompressed	220.21 KB	-	-
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	233.43 KB	-	-
@sentry/nextjs (client)	37.73 KB	+0.54%	+206 B 🔺
@sentry/sveltekit (client)	35.37 KB	-	-
@sentry/node	121.77 KB	+0.01%	+2 B 🔺
@sentry/node - without tracing	93.41 KB	-	-
@sentry/aws-serverless	103.11 KB	-	-

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