Updating API surface

[gauntface]

@pinarx pointed out that the types file was out of sync with the API surface.

@jshcrowthe we can do away with the types files if we get the "typescript-y-ness" of the source inline and up-to-date right, or is there something I'm missing? I.e. move away from closure and opt for typescript typing

[pinarx]

thank you!

[jshcrowthe]

This PR as it is looks fine. Really the larger issue here is that the @firebase/messaging and @firebase/messaging-types packages are entirely separate. In an ideal world you would have all of your implementation code, linked to your interfaces via implements statements. Going forward there will also be cases were components will need to be able to take dependencies on the interfaces provided by other components, w/o actually importing them directly.

I'd suggest that doing that would be well worth your time.

[gauntface]

Yeah we need to figure something out here to ensure it doesn't happen again.