Support of specific tmpDir with os.Root

Hi!

thanks for adding support for `os.Root()` here @stapelberg!

I would just have a small feature request:

- When using `WithRoot(r)` it is assumed by `renameio/v2` that tmp files should be in the top-level directory of `r`.
- In my case I would like to have them in a folder like `.sesam/tmp` and the destination after `CloseAtomicallyReplace()` would be somewhere other under `r`.
- Opening `.sesam/tmp` as own root does not help, as I can't reach the destination path then.
- The reason I want a dedicated `tmp` folder is that it can be cleared completely easier, opposed to several tmp files lying around (also that `.sesam/tmp` dir is automatically git-ignored).

Volunteering to add a PR, just thought I'd do a ticket first to see if that would be appreciated.
If yes, would that be a new option like `WithRootTempDir()`? Or would we honor `WithTempDir()` in that case by doing something like:

```go
tmpname = "." + filepath.Base(cfg.path)
if cfg.dir != "" {
tmpname = filepath.Join(cfg.dir, tmpname)
}
tmpname, f, err = openTempFileRoot(cfg.root, tmpname, cfg.createPerm)
```

(referenced line is [here](https://github.com/google/renameio/blob/v2.0.2/tempfile.go#L255))

Start in tempfile.go around the referenced openTempFileRoot call and trace how WithRoot and WithTempDir populate the configuration. Compare the proposed root-relative temporary directory behavior with CloseAtomicallyReplace’s destination handling. Done means temporary files can be placed under a configured directory within the root while replacement still reaches the requested destination, with coverage for the chosen API behavior.

Written by the indexing model from the issue text.