proposal: os: Export path escape error

### Proposal Details

When using `os.Root` the only way to detect when a path would escape the root is using string comparison on the error because [`errPathEscapes`](https://github.com/golang/go/blob/75a19dbcd7e69dc619fd57b9ed32f697121160ba/src/os/file.go#L421) isn't exported. I propose exporting `errPathEscapes` as `ErrPathEscapes` so that `errors.Is()` can be used.