feat(core): Allow to use continueTrace without callback

[mydea]

Inspired by recent comments about the continueTrace method, I figured we can actually update this in a backwards-compatible way to also allow to use it without a callback.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	65.61 KB (0%)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	55.72 KB (0%)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.01 KB (0%)
@sentry/browser - Webpack (gzipped)	21.32 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	62.04 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	29.13 KB (+0.03% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	21.27 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	195.72 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	88.39 KB (+0.02% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	63.35 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.86 KB (+0.03% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	65.92 KB (0%)
@sentry/react - Webpack (gzipped)	21.36 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	82.66 KB (0%)
@sentry/nextjs Client - Webpack (gzipped)	48.15 KB (0%)
@sentry-internal/feedback - Webpack (gzipped)	16.12 KB (0%)