Provide typescript definitions

It would be good to have some typescript definitions for this package. Right now i have needed to set `"noImplicitAny": false` in my `tsconfig.json' and also provide this hacky workaround, in my boot file (I am using Quasar on top of Vue):

```ts
declare module '@vue/runtime-core' {
  interface ComponentCustomProperties {
    $auth: any;
  }
}
```