feat(core): Ensure trace APIs always return a span

[mydea]

When tracing is disabled or otherwise skipped, they return a non recording span instead of undefined.

This aligns the startSpan, startSpanManual and startInactiveSpan APIs with opentelemetry, where you also always get a span back.

Especially, this makes our APIs much nicer to use, as you don't need to guard for span existence everywhere anymore 🎉

I tried to update all tests etc. to use this and remove some unnecessary check (side note: I wonder if we could add an eslint rule: https://typescript-eslint.io/rules/no-unnecessary-condition/) I'll look into maybe adding this rule as well.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.52 KB (+0.19% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	68.74 KB (+0.24% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	72.66 KB (+0.22% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.28 KB (+0.22% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.97 KB (+0.42% 🔺)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	32.97 KB (+0.42% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.36 KB (+0.06% 🔺)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.37 KB (+0.06% 🔺)
@sentry/browser - Webpack (gzipped)	22.57 KB (+0.06% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	75.58 KB (+0.21% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.18 KB (+0.19% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	33.06 KB (+0.45% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	23.98 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	211.04 KB (+0.24% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	99.84 KB (+0.51% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	71.67 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	36.22 KB (+0.47% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	69 KB (+0.2% 🔺)
@sentry/react - Webpack (gzipped)	22.6 KB (+0.06% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	85.44 KB (+0.18% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	49.84 KB (+0.28% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	17.41 KB (0%)

[AbhiPrasad]

I'm gonna merge this in because I need it for a change