fix(build): add observable-media-service to index.ts

[karlhaas]

After upgrade to beta 4, I can't import ObservableMediaService from the barrel.

fixes #144.

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[karlhaas]

I signed it!

[googlebot]

CLAs look good, thanks!

[ThomasBurleson]

@karlhaas - good catch.

If needed before Beta5, you can alway use the full path reference to ObservableMediaService.

[ThomasBurleson]

@karlhaas - please squash and update the commit with title:

fix(build): add observable-media-service to exported barrel

[karlhaas]

@ThomasBurleson done

[ThomasBurleson]

@karlhaas - We love community contributions. Thx again.

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