As adding suspense: true directly to the useSWR call removes the undefined from the data type, enabling more precise type inference. However, this only works when suspense: true is explicitly passed to each useSWR call — it does not take effect when configured globally via <SWRConfig value={{ suspense: true }}>.

// just using swr or even with global configuration
const { data } = useSWR<Data>('/key', fetcher)
//      ^ Data | undefined

const { data } = useSWR<Data>('/key', fetcher, { suspense: true })
//      ^ Data

This PR add the new SWRGlobalConfig interface allows customizing type inference through declaration merging:

declare module 'swr' {
  interface SWRGlobalConfig {
    suspense: true
  }
}

const { data } = useSWR<Data>('/key', fetcher)
//      ^ Data

As Suspense becomes more common, we want type inference to stay accurate.

Ref #266 and closes #1412