feat(nextjs): Warn when client was initialized more than once

[lforst]

With the addition on instrumentation-client.ts the likelihood of Sentry.init() rose starkly so I figured it might be worth printing a warning.

Resolves getsentry/sentry-wizard#930

[github-actions]

size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	23.2 KB	-	-
@sentry/browser - with treeshaking flags	23.02 KB	-	-
@sentry/browser (incl. Tracing)	36.83 KB	-	-
@sentry/browser (incl. Tracing, Replay)	73.99 KB	-	-
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	67.39 KB	-	-
@sentry/browser (incl. Tracing, Replay with Canvas)	78.66 KB	-	-
@sentry/browser (incl. Tracing, Replay, Feedback)	91.22 KB	-	-
@sentry/browser (incl. Feedback)	40.33 KB	-	-
@sentry/browser (incl. sendFeedback)	27.83 KB	-	-
@sentry/browser (incl. FeedbackAsync)	32.63 KB	-	-
@sentry/react	25 KB	-	-
@sentry/react (incl. Tracing)	38.75 KB	-	-
@sentry/vue	27.41 KB	-	-
@sentry/vue (incl. Tracing)	38.55 KB	-	-
@sentry/svelte	23.23 KB	-	-
CDN Bundle	24.44 KB	-	-
CDN Bundle (incl. Tracing)	36.85 KB	-	-
CDN Bundle (incl. Tracing, Replay)	71.87 KB	-	-
CDN Bundle (incl. Tracing, Replay, Feedback)	77.07 KB	-	-
CDN Bundle - uncompressed	71.24 KB	-	-
CDN Bundle (incl. Tracing) - uncompressed	108.94 KB	-	-
CDN Bundle (incl. Tracing, Replay) - uncompressed	220.23 KB	-	-
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	232.8 KB	-	-
@sentry/nextjs (client)	40.21 KB	+0.38%	+153 B 🔺
@sentry/sveltekit (client)	37.26 KB	-	-
@sentry/node	142.89 KB	-	-
@sentry/node - without tracing	96.08 KB	-	-
@sentry/aws-serverless	120.43 KB	+0.01%	+4 B 🔺

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