feat(core): Add name to Span

[mydea]

In order to align Span & Transaction for usage in new performance APIs, this add name to the Span that also always returns a string.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	75.35 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.24 KB (+0.03% 🔺)
@sentry/browser - Webpack (gzipped)	21.84 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	69.87 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.18 KB (+0.05% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	20.18 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	220.81 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	85.09 KB (+0.08% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	59.73 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.08 KB (+0.03% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	75.37 KB (+0.02% 🔺)
@sentry/react - Webpack (gzipped)	21.88 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	93.23 KB (+0.02% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	50.81 KB (+0.03% 🔺)