feat(nextjs): Add captureRouterTransitionStart hook for capturing navigations

[lforst]

We relied on Next.js internals for navigation spans and that inevitably broke so Next.js added a onRouterTransitionStart hook in instrumentation-client.ts (vercel/next.js#77791) for us to use.

This PR exposes a handler called captureRouterTransitionStart for that hook so that we can actually instrument navigations again.

[github-actions]

size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	23.26 KB	-	-
@sentry/browser - with treeshaking flags	23.09 KB	-	-
@sentry/browser (incl. Tracing)	36.91 KB	-	-
@sentry/browser (incl. Tracing, Replay)	74.11 KB	-	-
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	67.48 KB	-	-
@sentry/browser (incl. Tracing, Replay with Canvas)	78.77 KB	-	-
@sentry/browser (incl. Tracing, Replay, Feedback)	90.58 KB	-	-
@sentry/browser (incl. Feedback)	39.65 KB	-	-
@sentry/browser (incl. sendFeedback)	27.87 KB	-	-
@sentry/browser (incl. FeedbackAsync)	32.65 KB	-	-
@sentry/react	25.06 KB	-	-
@sentry/react (incl. Tracing)	38.83 KB	-	-
@sentry/vue	27.47 KB	-	-
@sentry/vue (incl. Tracing)	38.64 KB	-	-
@sentry/svelte	23.29 KB	-	-
CDN Bundle	24.5 KB	-	-
CDN Bundle (incl. Tracing)	36.94 KB	-	-
CDN Bundle (incl. Tracing, Replay)	71.99 KB	-	-
CDN Bundle (incl. Tracing, Replay, Feedback)	77.15 KB	-	-
CDN Bundle - uncompressed	71.45 KB	-	-
CDN Bundle (incl. Tracing) - uncompressed	109.21 KB	-	-
CDN Bundle (incl. Tracing, Replay) - uncompressed	220.5 KB	-	-
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	233.03 KB	-	-
@sentry/nextjs (client)	40.46 KB	+0.29%	+116 B 🔺
@sentry/sveltekit (client)	37.35 KB	-	-
@sentry/node	142.92 KB	-	-
@sentry/node - without tracing	96.13 KB	+0.01%	+1 B 🔺
@sentry/aws-serverless	120.47 KB	-0.01%	-1 B 🔽

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[AbhiPrasad]

lol i guess that makes sense