enhance: add $ prefix for non-major keys

[huozhi]

specialize keys for error and isValidating, and some non major keys in swr infinite, to identify them easier later if developers want to play tricks with keys or filter them from cache provider.

Resolves #242

e.g.

access all keys

const provider = new Map()
const { cache, mutate } = createCache(provider)

// ...
const serializedKeys = Array.from(provider.keys()).filter(k => !k.startsWith('$'))

access pending requests

const validatingKeys = serializedKeys.filter(key => key.startsWith('$req$'))

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SWR-SSR	Configuration

[shuding]

Let's also add it to inf, len and ctx in swr-infinite 👍

[huozhi]

curious about the reason of designing page key as firstPageKey ? ['$inf', firstPageKey] : null, then the array key will always be serialized into arg@blablabla which losing the $ prefix

[shuding]

Oh that's a good point, since it already has $args.

curious about the reason of designing page key as firstPageKey ? ['$inf', firstPageKey] : null

That's because if we directly use firstPageKey as the key (without adding inf), the infinite page hook will share cache with the single page hook if it exists (useSWR(firstPageKey)).

[shuding]

Looks good to me, let's also change existing @ concatenation symbols to $ so it looks better :)

[shuding]

make symbol rich

Thanks! 🤣