feat(browser): Export makeMultiplexedTransport from browser SDK

[AbhiPrasad]

ref #7926

This is meant to be used in the browser, so let's export this here!

Exporting this from index.ts instead of exports.ts because we don't want this in the CDN bundles.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	21.02 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	65.66 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.56 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified)	58.12 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	21.17 KB (0%)
@sentry/browser - Webpack (minified)	69.07 KB (0%)
@sentry/react - Webpack (gzipped + minified)	21.19 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	49.05 KB (+0.03% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	28.63 KB (+0.1% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.84 KB (+0.05% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	46.87 KB (+0.15% 🔺)
@sentry/replay - Webpack (gzipped + minified)	40.67 KB (+0.19% 🔺)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	65.71 KB (+0.13% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	58.64 KB (+0.13% 🔺)