Add isFallback state

[shuding]

After putting some time into #1922, I got the conclusion that:

• The fallback logic should be inside the useSWR hook, and middleware should receive the same final value as the end user.
• To correctly implement some logic, not limiting to laggy, both middleware and the user should be able to tell if the fallback is returned.

And in 2.0, fallback will be an important feature for SWR as it covers all the SSR cases (Suspense in SSR too). It's critical to make it more complete.

[codesandbox-ci]

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Latest deployment of this branch, based on commit d710ddd:

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