[typings][Typescript] Fix TS definitions so modules can be accessed from the default export

[pheromonez]

Fixes #774

Have kept this commit small and only touched the parts of the code that needed to change in order to resolve the linked issue. Can now access auth (and other modules) using this syntax:

import firebase from 'react-native-firebase';
firebase.auth().signInAnonymously().then(........);

The current method of access is still supported:

import firebase from 'react-native-firebase';
firebase.app().auth().signInAnonymously().then(........);

[chrisbianca]

This looks great, thank you!

@Ehesp @Salakar if you're happy we can get this merged in.

[chrisbianca]

@pheromonez I've merged #771 which has thrown up a merge conflict here. Are you able to resolve and then we can get this merged too? Thanks again!

[chrisbianca]

@pheromonez Thanks for the tweaks. Has this resolved all of the compilation issues that were flagged by @danielw93 as part of #771? If so, we can get this merged...

[pheromonez]

Just pushed another change, somehow two of the fixes that @danielw93 put through in #784 got reverted. I probably merged incorrectly. Anyhow, my local Typescript compiler isn't complaining about this file so I think it's good to go :)

[Ehesp]

Thanks again for this!

[pheromonez]

@Ehesp Happy to help, thanks for the great library, it's saved us heaps on our current project