fix(nextjs): Export BrowserTracing integration directly

[lobsterkatie]

This applies the changes made in #4204 (exporting BrowserTracing outside of Integrations, in order that it be treeshakable if necessary) to the nextjs SDK, which got left out of that PR.

Fixes#4411.

[github-actions]

size-limit report

Path	Base Size (86fa701)	Current Size	Change
@sentry/browser - CDN Bundle (gzipped)	19.68 KB	19.68 KB	+0.01% 🔺
@sentry/browser - CDN Bundle (minified)	62.89 KB	62.9 KB	+0.01% 🔺
@sentry/browser - Webpack	22.21 KB	22.21 KB	0%
@sentry/browser - Webpack - gzip = false	76.02 KB	76.02 KB	0%
@sentry/react - Webpack	22.24 KB	22.24 KB	0%
@sentry/nextjs Client - Webpack	46.23 KB	46.23 KB	0%
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	28.22 KB	28.23 KB	+0.02% 🔺