feat: Export spanToBaggageHeader utility

[mydea]

So we have a proper usage path without installing utils & core...

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser	21.65 KB (0%)
@sentry/browser (incl. Tracing)	32.67 KB (-0.05% 🔽)
@sentry/browser (incl. Tracing, Replay)	68.01 KB (-0.04% 🔽)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	61.41 KB (-0.04% 🔽)
@sentry/browser (incl. Tracing, Replay with Canvas)	72.05 KB (-0.04% 🔽)
@sentry/browser (incl. Tracing, Replay, Feedback)	84.18 KB (-0.03% 🔽)
@sentry/browser (incl. Feedback)	37.69 KB (0%)
@sentry/browser (incl. sendFeedback)	26.21 KB (0%)
@sentry/browser (incl. FeedbackAsync)	30.77 KB (0%)
@sentry/react	24.33 KB (0%)
@sentry/react (incl. Tracing)	35.63 KB (-0.05% 🔽)
@sentry/vue	25.48 KB (0%)
@sentry/vue (incl. Tracing)	34.46 KB (-0.04% 🔽)
@sentry/svelte	21.78 KB (0%)
CDN Bundle	24.14 KB (+0.46% 🔺)
CDN Bundle (incl. Tracing)	34.04 KB (-0.03% 🔽)
CDN Bundle (incl. Tracing, Replay)	67.71 KB (-0.03% 🔽)
CDN Bundle (incl. Tracing, Replay, Feedback)	72.84 KB (-0.02% 🔽)
CDN Bundle - uncompressed	70.99 KB (+0.5% 🔺)
CDN Bundle (incl. Tracing) - uncompressed	100.98 KB (-0.03% 🔽)
CDN Bundle (incl. Tracing, Replay) - uncompressed	210.59 KB (-0.02% 🔽)
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	223.64 KB (-0.02% 🔽)
@sentry/nextjs (client)	34.86 KB (-0.04% 🔽)
@sentry/sveltekit (client)	33.23 KB (-0.04% 🔽)
@sentry/node	138.72 KB (0%)