feat(core): Add `startNewTrace` API

[Lms24]

This PR adds a new Sentry.startNewTrace function that allows users to start a trace in isolation of a potentially still active trace. When this function is called, a new trace will be started on a forked scope which remains valid throughout the callback lifetime.

Simple usage example:

myButton.addEventListener('click', async () => {
  Sentry.startNewTrace(() => {
    Sentry.startSpan({ op: 'ui.interaction.click', name: 'fetch click' }, async () => {
      await fetch('http://example.com');
    });
  });
});

Extraced the generatePropagationContext utility to utils because it's used in multiple packages

[github-actions[bot]]

size-limit report 📦

Path	Size
@sentry/browser	21.74 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing)	32.75 KB (-0.06% 🔽)
@sentry/browser (incl. Tracing, Replay)	68.22 KB (-0.04% 🔽)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	61.64 KB (-0.03% 🔽)
@sentry/browser (incl. Tracing, Replay with Canvas)	72.27 KB (-0.03% 🔽)
@sentry/browser (incl. Tracing, Replay, Feedback)	84.31 KB (-0.02% 🔽)
@sentry/browser (incl. Feedback)	37.75 KB (-0.01% 🔽)
@sentry/browser (incl. sendFeedback)	26.31 KB (-0.02% 🔽)
@sentry/browser (incl. FeedbackAsync)	30.73 KB (-0.01% 🔽)
@sentry/react	24.43 KB (+0.01% 🔺)
@sentry/react (incl. Tracing)	35.75 KB (-0.06% 🔽)
@sentry/vue	25.68 KB (-0.01% 🔽)
@sentry/vue (incl. Tracing)	34.56 KB (-0.08% 🔽)
@sentry/svelte	21.88 KB (+0.02% 🔺)
CDN Bundle	24.28 KB (+0.01% 🔺)
CDN Bundle (incl. Tracing)	34.22 KB (+0.01% 🔺)
CDN Bundle (incl. Tracing, Replay)	68.04 KB (+0.02% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback)	73.04 KB (+0.04% 🔺)
CDN Bundle - uncompressed	71.46 KB (+0.01% 🔺)
CDN Bundle (incl. Tracing) - uncompressed	101.58 KB (+0.04% 🔺)
CDN Bundle (incl. Tracing, Replay) - uncompressed	211.48 KB (+0.02% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	223.84 KB (+0.02% 🔺)
@sentry/nextjs (client)	35.1 KB (-0.04% 🔽)
@sentry/sveltekit (client)	33.35 KB (-0.04% 🔽)
@sentry/node	114.33 KB (+0.01% 🔺)
@sentry/aws-serverless	103.2 KB (+0.01% 🔺)