feat: add isLoadingMore option for useSWRInfinite

[devjiwonchoi]

The initiative of isLoadingMore feature.

Description

Since isLoading is limited to the initial fetch, need a custom case for load more in infinite loading.

Didn't modify the current isLoading due to this comment:

swr/core/src/use-swr.ts

Lines 379 to 383 in bf1cd03

	// It is in the `isLoading` state, if and only if there is no cached data.
	// This bypasses fallback data and laggy data.
	if (isUndefined(getCache().data)) {
	initialState.isLoading = true
	}

Reference

https://swr.vercel.app/examples/infinite-loading

Fixes:
#2410

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 5fd4744:

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

[devjiwonchoi]

@shuding Respectfully requesting for a review.