Support synchronous fetcher function

[rauchg]

For local storage cases like localStorage, I ran into an obscure error where it was complaining that the return value didn't have .catch. I fixed by turning my sync fetcher into an async function.

Furthermore, it might be worth considering accepting the fetcher as the second param, since that's how SWR also works.

[rauchg]

(Great work on sswr btw! Enjoying it 🤗 )

[ConsoleTVs]

@rauchg I quickly checked the types for the fetcher function:

type SWRFetcher<D = any> = (...props: any[]) => Promise<D>;

I'm curious how it allowed you to use a fetcher function that didn't return a Promise. I will cover this use case, but may I know if you were using Typescript and/or vscode?

[ConsoleTVs]

Fixed here: ConsoleTVs/swrev@815d5ee