ref: Make setupOnce optional in integrations

[mydea]

This is now properly optional 🎉

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.83 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	69.08 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	73 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.66 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	33.27 KB (+0.05% 🔺)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	33.18 KB (+0.04% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.16 KB (+0.06% 🔺)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.16 KB (+0.05% 🔺)
@sentry/browser - Webpack (gzipped)	22.42 KB (+0.07% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	76.19 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.7 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	33.61 KB (+0.06% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	24.73 KB (+0.08% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	212.98 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	101.61 KB (-0.02% 🔽)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	74.1 KB (-0.02% 🔽)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	36.87 KB (+0.02% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	69.42 KB (+0.01% 🔺)
@sentry/react - Webpack (gzipped)	22.45 KB (+0.07% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	86.88 KB (+0.01% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	50.09 KB (+0.01% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	17.14 KB (0%)