ref(core): Make on and emit required on client

[mydea]

This allows us to remove a bunch of code around fallbacks etc. Esp. in replay this also allows us to remove a bunch of code around breadcrumbs etc.

note @JonasBa I did not remove this in profiling-node yet because the test rely on this quite a bunch, can you take a look at this when you've got some time?

[Lms24]

Finally 🚀

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.86 KB (-0.39% 🔽)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	69.09 KB (-0.41% 🔽)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	73.02 KB (-0.41% 🔽)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.72 KB (-0.47% 🔽)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	33.25 KB (-0.32% 🔽)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	33.1 KB (-0.36% 🔽)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.13 KB (-0.21% 🔽)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.13 KB (-0.25% 🔽)
@sentry/browser - Webpack (gzipped)	22.39 KB (-0.4% 🔽)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	75.87 KB (-0.43% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.4 KB (-0.46% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	33.34 KB (-0.4% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped)	24.45 KB (-0.47% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	212.42 KB (-0.59% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	100.97 KB (-0.5% 🔽)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	73.38 KB (-0.65% 🔽)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	36.52 KB (-0.33% 🔽)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	69.47 KB (-0.43% 🔽)
@sentry/react - Webpack (gzipped)	22.44 KB (-0.36% 🔽)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	87.21 KB (-0.37% 🔽)
@sentry/nextjs Client - Webpack (gzipped)	50.32 KB (-0.21% 🔽)
@sentry-internal/feedback - Webpack (gzipped)	17.11 KB (-0.55% 🔽)