feat(browser): Use fetch keepalive flag

[lforst]

While working on #5676 I noticed some navigation transactions don't show up in Sentry. I found out that when navigating between pages, Chrome cancels any outgoing requests - including requests to Sentry.

Events not being sent because of a page navigation probably happens the most with navigation transactions, since we usually finish the transaction right before we navigate (finishing happens implicitly on navigation). To requests being cancelled, we can use fetch's keepalive flag to keep outgoing requests alive, even when we're switching pages.

#2547 has an excellent writeup on the pros and cons of this flag. In short:

• keepalive isn't supported by Firefox
• As per specification, a fetch request with keepalive: true and a content length of > 64 kibibytes returns a network error

This change definitely isn't a solution to solve this problem in 100% of the cases but it is our best effort to improve the status quo.

---

Resolves #2547

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.43 KB (+0.06% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	60.11 KB (+0.06% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18 KB (+0.07% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	52.97 KB (+0.06% 🔺)
@sentry/browser - Webpack (gzipped + minified)	19.8 KB (+0.07% 🔺)
@sentry/browser - Webpack (minified)	64.36 KB (+0.05% 🔺)
@sentry/react - Webpack (gzipped + minified)	19.82 KB (+0.08% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	44.68 KB (+0.03% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.93 KB (+0.05% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.3 KB (+0.06% 🔺)