ref(core): Split transaction finish into underlying _finishTransaction

[mydea]

This makes it easier to extend the transaction (e.g. for POTEL) and make use of the finishing code without actually sending the transaction to sentry.

This is needed for #9085.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	84.26 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.42 KB (+0.05% 🔺)
@sentry/browser - Webpack (gzipped)	22 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	78.69 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.52 KB (+0.07% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	20.59 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	254.14 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	86.42 KB (+0.05% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	61.23 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.38 KB (+0.03% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	84.28 KB (+0.01% 🔺)
@sentry/react - Webpack (gzipped)	22.03 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	102.2 KB (-0.01% 🔽)
@sentry/nextjs Client - Webpack (gzipped)	50.95 KB (+0.01% 🔺)