Improve the pagination API

[shuding]

After this change, the SWR -> offset function will pass the entire SWR object (so you can get the error / loading state as well), as well as the index of current page. (pageSWRs could be overkill and useless, and it causes some data inconsistency issues.)

Before

swrDataToOffset(data, pageSWRs)

e.g.:

useSWRPages(
  ...,
  ...,
  (data, swrs) => offsetOfNextPage
)

After

SWRToOffset(SWR, index)

e.g.:

useSWRPages(
  ...,
  ...,
  ({ data, error, ... }, indexOfCurrentPage) => offsetOfNextPage
)

Also added an example for it.