Add .fs() method

[sindresorhus]

Same as the existing method, but checks if the paths exists and also follows symlinks. This could be used when you know both path exists. It could potentially also be an option.

// @chrisblossom Thoughts?

[sholladay]

Wouldn't this just be the same as isPathInside(fs.realpathSync(a), fs.realpathSync(b))?

In my opinion, it definitely deserves a place in the docs, but it doesn't feel worth a new API.