feat(core): Implement startIdleSpan

[mydea]

For now, this does not actually use this anywhere.

The idle span works mostly like the idle transaction, but there are small differences:

1. I replaced the heartbeat with simply waiting for 15s (instead of 3x5s) after the last span was started. IMHO semantically this is good enough, and makes it easier to reason about this? LMK if I am missing something there.
2. This means that the idle span ends when either:
   a. No span was created for 1s
   b. The last time a span was created was 15s ago
   c. 30s passed (final timeout)
3. Instead of the delayAutoFinishUntilSignal we used before for e.g. Next.js streaming pages, we now have disableAutoFinish, together with client.emit('idleSpanEnableAutoFinish', idleSpan)
4. Idle spans are always on the scope (we never used it without this setting)
5. You can directly pass beforeSpanEnd (instead of registerBeforeFinishCallback()), which we can use for web vital collection etc.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.45 KB (+0.1% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	68.64 KB (+0.09% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	72.56 KB (+0.09% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.21 KB (+0.1% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.9 KB (+0.18% 🔺)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	32.9 KB (+0.18% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.34 KB (0%)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.35 KB (0%)
@sentry/browser - Webpack (gzipped)	22.56 KB (0%)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	75.53 KB (+0.13% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.12 KB (+0.11% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	32.99 KB (+0.25% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	23.97 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	210.74 KB (+0.1% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	99.55 KB (+0.21% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	71.65 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	36.15 KB (+0.27% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	68.93 KB (+0.09% 🔺)
@sentry/react - Webpack (gzipped)	22.59 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	85.35 KB (+0.07% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	49.76 KB (+0.12% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	17.41 KB (0%)