fix: Add clear method to Cache interface

[myarete]

https://swr.vercel.app/docs/advanced/cache#access-to-the-cache

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[shuding]

Thanks for the PR! clear is currently not a strong requirement for the cache provider, as we don't intend to require every method of Map to be implemented as the custom cache but only the very basic ones. I think a type-safe way to do this is:

if (cache instanceof Map) cache.clear()

[piotr-cz]

Anyway this method is described in Cache docs:

cache.clear() // ⚠️ Clear all the cache. SWR will revalidate upon re-render.

To maintain backward compatibility with custom Cache Provider implementations (which are not an instances of Map), I'd suggest adding the clear method` as an optional method:

export interface Cache<Data = any> {
    // ...
    clear?(): void;
}

BTW methods has been removed from interface in b38d1bd#diff-c54113cf61ec99691748a3890bfbeb00e10efb3f0a76f03a0fd9ec49072e410aL217