feat: Ensure getRootSpan() does not rely on transaction

[mydea]

Instead, we keep the root span as a non-enumerable property on the span. We use the already existing method for this, so this should work reasonably well.

This also means that now getRootSpan can always return a Span, not undefined anymore, which makes types a bit easier there.

Also, we now ensure to export this everywhere.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	68.26 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	72.14 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	61.82 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.46 KB (+0.09% 🔺)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	32.46 KB (+0.09% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.37 KB (+0.04% 🔺)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.38 KB (+0.04% 🔺)
@sentry/browser - Webpack (gzipped)	22.56 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback) - CDN Bundle (gzipped)	75.11 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing, Replay) - CDN Bundle (gzipped)	66.73 KB (+0.07% 🔺)
@sentry/browser (incl. Tracing) - CDN Bundle (gzipped)	32.57 KB (+0.15% 🔺)
@sentry/browser - CDN Bundle (gzipped)	24 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing, Replay) - CDN Bundle (minified & uncompressed)	209.11 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing) - CDN Bundle (minified & uncompressed)	97.92 KB (+0.1% 🔺)
@sentry/browser - CDN Bundle (minified & uncompressed)	71.68 KB (-0.06% 🔽)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	68.46 KB (+0.04% 🔺)
@sentry/react - Webpack (gzipped)	22.59 KB (+0.05% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	84.95 KB (+0.01% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	49.36 KB (+0.05% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	17.41 KB (+0.03% 🔺)