feat(tracing): Add build dir in npm package

[linguofeng]

ref https://github.com/getsentry/sentry-javascript/blob/master/packages/browser/.npmignore

https://unpkg.com/browse/@sentry/browser/build/ is Found

https://unpkg.com/browse/@sentry/tracing/build/ is NotFound

Before submitting a pull request, please take a look at our
Contributing guidelines and verify:

• 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).

[kamilogorek]

Good catch. Thanks!