fix(tracing): Expose startTransaction in CDN bundle (#2938)

Make sure it appears on `window.Sentry` as well as `window.__SENTRY__.extensions`, so it's accessible in the public API. (Because of how things are built, this only affected the bundle, not the npm package.)

packages/tracing/src/index.bundle.ts

@@ -30,6 +30,7 @@ export {
   setTag,
   setTags,
   setUser,
+  startTransaction,
   Transports,
   withScope,
 } from '@sentry/browser';