feat: add the parallel option in useSWRInfinite

[koba04]

This PR adds the parallel option into useSWRInfinite to make revalidations in parallel.

useSWRInfinite(getKey, fetcher, { parallel: true })

The default value is false.

Motivation

When I revalidate multiple pages with useSWRInfinite, useSWRInfinte sends the revalidation requests sequentially, which is slow when we have many pages to revalidate. This especially becomes a problem when revalidations happen frequently.

// without parallel
page1 ====> page2 ====> page3 ====> done

// parallel
page1
page2 ====> done 
page3

Limitation

The parallel option doesn't work with cursor-based APIs because cursor-based APIs send the next request based on the cursor value returned from the current request.

https://swr.vercel.app/docs/pagination#example-2-cursor-or-offset-based-paginated-api

With the parallel option, useSWRInfinite can't get the previousPageData because requests happen in parallel. The current implementation passes null as previousPageData with the parallel option.
TODO: infer the type of previousPageData based on the parallel option.

Alternative

I've also considered the concurrency = 1 option to be able to handle revalidations more flexibly. But I had to handle previousPageData differently based on whether revalidations are sequential or parallel. I think the parallel option is not more flexible than the concurrency option, but it's clearer than the concurrency option.

[codesandbox-ci]

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Latest deployment of this branch, based on commit 5ee0783:

Sandbox	Source
SWR-Basic	Configuration
SWR-States	Configuration
SWR-Infinite	Configuration
SWR-SSR	Configuration

[shuding]

I like this idea, so if parallel = true this works like a list for dynamic number of requests right? It seems that this covers the use case for "useSWRList" too

[koba04]

Yeah, that sounds great👍 This still has a concept of page, so some cases might not fit this though.