fs: add errback style support to exists()

`fs.exists()` does not currently support Node's standard callback signature. This commit adds errback support while maintaining backwards compatibility. Closes #8369.
That performance hit is ugly. There is no point in checking the node version though. I guess there are a few options.
- Do nothing and close this PR. This function isn't recommended for use anyway.
- Take the performance hit, maintain backward compatibility, and support both styles (current implementation).
- Introduce a breaking change and support only the errback style.