TS errors if model methods are not async

[z1haze]

According to the docs model methods are supposed to support either sync or async, but in practice when I attempted to, for example, write the generateRefreshToken in a synchronous function, typescript errors because it isn'y async. So i peaked into the interface that the source validates against, and all of the methods are written to return a promise.

Documentation stating that they can be sync (supposedly)

Version >=5.x: Callback support has been removed! Each model function supports either sync or async (Promise or async function) return values.

Source showing it expects it to return a promise

generateRefreshToken?(client: Client, user: User, scope: string[]): Promise<string>;

[jankapunkt]

@z1haze thanks for reporting this. I think the docs are just confusing here. The lib treats every model function as async, which in turn of course allows to also pass sync methods, as they are resolved within an async context and are thus non-blocking.

I created a PR as initial input for further discussion: #347