ref(core): Update multiplexed transport to default to errors only

[mydea]

By default, we only multiplex errors, but allow to opt-in to more envelope types.

After talking about this with @AbhiPrasad , this is actually the default we want to provide (for now).

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	21.02 KB (0%)
@sentry/browser - ES5 CDN Bundle (minified)	65.66 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.56 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	58.12 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	21.17 KB (0%)
@sentry/browser - Webpack (minified)	69.07 KB (0%)
@sentry/react - Webpack (gzipped + minified)	21.19 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	49.04 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	28.59 KB (+0.01% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.82 KB (0%)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	46.55 KB (+0.16% 🔺)
@sentry/replay - Webpack (gzipped + minified)	40.36 KB (+0.23% 🔺)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	65.33 KB (+0.11% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	58.32 KB (+0.13% 🔺)