fix(loader): Ensure JS loader works with tracing & add tests

[mydea]

This adds some new integration tests to test the JS loader, making sure the behavior is actually as expected.
This is especially important for the newer loader variants, potentially including tracing & replay.

Some small adjustments to tracing were necessary to ensure it works together with the loader.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	20.66 KB (+0.22% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	64.53 KB (+0.21% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.2 KB (+0.25% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	56.91 KB (+0.24% 🔺)
@sentry/browser - Webpack (gzipped + minified)	21.57 KB (+0.19% 🔺)
@sentry/browser - Webpack (minified)	72.13 KB (+0.19% 🔺)
@sentry/react - Webpack (gzipped + minified)	21.6 KB (+0.2% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	52.09 KB (+0.08% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	28.26 KB (+0.18% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.45 KB (+0.18% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	44.78 KB (+0.1% 🔺)
@sentry/replay - Webpack (gzipped + minified)	38.86 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	63.5 KB (+0.08% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	56.54 KB (+0.08% 🔺)