new feature: add revalidateReason to indicate which reason to cause revalidate.

[lovetingyuan]

In SWR, many factors can trigger revalidation. I'd like to know the specific reason for each revalidation, such as focus, error retry, or interval, etc.

const { data, isLoading, isValidating, validateReason } = useSWR()

when validateReason is updated, I know the new data is fetched and I also know why the api is refeched.

[ggualiato]

This is something I've been missing sometimes. For example, cases where my query returns data for different parts of the UI, but when I revalidate, I want to show a different UI depending on where I triggered the revalidation.