WIP: Add finish immediate to buffer

[mydea]

Just a draft to try this out

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.82 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	61.47 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.5 KB (+0.02% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	54.77 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.21 KB (0%)
@sentry/browser - Webpack (minified)	66.17 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.24 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.48 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.74 KB (0%)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.03 KB (+0.02% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	43.28 KB (-0.46% 🔽)
@sentry/replay - Webpack (gzipped + minified)	38.06 KB (-0.41% 🔽)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	60.55 KB (-0.3% 🔽)

[mydea]

Actual implementation by billy: #6854