ref(core): Streamline global handlers & add comments

[mydea]

This adds a comment explaining why we are using window.onerror instead of window.addEventListener('error'), for future reference.

It also removes the check for __SENTRY_LOADER__ check, which we removed quite some time ago in the Loader.

[github-actions]

size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	23.19 KB	added	added
@sentry/browser - with treeshaking flags	21.87 KB	added	added
@sentry/browser (incl. Tracing)	35.69 KB	added	added
@sentry/browser (incl. Tracing, Replay)	72.59 KB	added	added
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	63.02 KB	added	added
@sentry/browser (incl. Tracing, Replay with Canvas)	76.9 KB	added	added
@sentry/browser (incl. Tracing, Replay, Feedback)	89.39 KB	added	added
@sentry/browser (incl. Feedback)	39.95 KB	added	added
@sentry/browser (incl. sendFeedback)	27.8 KB	added	added
@sentry/browser (incl. FeedbackAsync)	32.59 KB	added	added
@sentry/react	25.87 KB	added	added
@sentry/react (incl. Tracing)	38.5 KB	added	added
@sentry/vue	27.4 KB	added	added
@sentry/vue (incl. Tracing)	37.51 KB	added	added
@sentry/svelte	23.35 KB	added	added
CDN Bundle	24.36 KB	added	added
CDN Bundle (incl. Tracing)	37.34 KB	added	added
CDN Bundle (incl. Tracing, Replay)	72.21 KB	added	added
CDN Bundle (incl. Tracing, Replay, Feedback)	77.55 KB	added	added
CDN Bundle - uncompressed	71.6 KB	added	added
CDN Bundle (incl. Tracing) - uncompressed	110.88 KB	added	added
CDN Bundle (incl. Tracing, Replay) - uncompressed	223.95 KB	added	added
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	237.17 KB	added	added
@sentry/nextjs (client)	38.79 KB	added	added
@sentry/sveltekit (client)	36.17 KB	added	added
@sentry/node	152.1 KB	added	added
@sentry/node - without tracing	98.98 KB	added	added
@sentry/aws-serverless	144 KB	added	added