feat: Export spanToJSON and spanToTraceHeader from all relevant packages

[lforst]

I was looking to bump the docs to v8, but it seems like spanToJSON and spanToTraceHeader were missed in a few packages (including Next.js).

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback)	75.65 KB (0%)
@sentry/browser (incl. Tracing, Replay)	66.87 KB (0%)
@sentry/browser (incl. Tracing, Replay with Canvas)	70.73 KB (0%)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	60.44 KB (0%)
@sentry/browser (incl. Tracing)	31.75 KB (0%)
@sentry/browser (incl. browserTracingIntegration)	31.75 KB (0%)
@sentry/browser (incl. feedbackIntegration)	30.88 KB (0%)
@sentry/browser (incl. feedbackModalIntegration)	30.9 KB (0%)
@sentry/browser (incl. feedbackScreenshotIntegration)	30.9 KB (0%)
@sentry/browser (incl. sendFeedback)	26.89 KB (0%)
@sentry/browser	22.1 KB (0%)
CDN Bundle (incl. Tracing, Replay, Feedback)	71.64 KB (+0.02% 🔺)
CDN Bundle (incl. Tracing, Replay)	66.39 KB (+0.02% 🔺)
CDN Bundle (incl. Tracing)	32.71 KB (+0.03% 🔺)
CDN Bundle	24.2 KB (+0.3% 🔺)
CDN Bundle (incl. Tracing, Replay) - uncompressed	207.01 KB (+0.02% 🔺)
CDN Bundle (incl. Tracing) - uncompressed	97.61 KB (+0.03% 🔺)
CDN Bundle - uncompressed	71.56 KB (+0.28% 🔺)
@sentry/react (incl. Tracing, Replay)	66.99 KB (0%)
@sentry/react	22.12 KB (0%)