Add test to ensure typescript users can use this module

[ofrobots]

Originally reported by @dauledk on #177 (comment).

However I still get a ton of compile errors when building. A lot of them comes from the google-auth-library. Like this one

<full-path-to-project>/node_modules/google-auth-library/types/lib/transporters.d.ts(1,23): error TS2688: Cannot find type definition file for 'request'.`

[ofrobots]

@dauledk tracked down the reason to an bug in types in one of our dependencies: #179. That issue can be used to investigate switching dependencies.

Let's use this bug for adding a test to make sure our type definitions are actually usable by users.