build(browser): Add function export in minified bundle for micro frontends

[philippeVV]

Add the function makeMultiplexedTransport from @sentry/core to the export in the browser package.
Make the function available in the browser minified bundle.

This function is required to use Sentry Micro Frontend.

• If you've added code that should be tested, please add tests.
• Ensure your code lints and the test suite passes (yarn lint) & (yarn test).

[Lms24]

Hey @philippeVV, thanks for opening this PR! makeMultiplexedTransport is currently only exported from the NPM package, you're right. But there's a reason for it: Bundle size would increase for all CDN bundle users, while multiplexed transport usage is rather the exception than the norm. Before we merge this, I'd like to check if we can find an alternative solution. Is there a way you could use the NPM package instead of the CDN bundles?