Create directory if not exists, and return `Dir`

Is there a simple way to create a directory if it does not yet exist and to return the `Dir`?

Something along the lines of:

```rs
async fn create_dir(parent: &Dir, path: &str) -> anyhow::Result {
match parent.create_dir(path) {
Ok(_) => {}
Err(e) => match e.kind() {
std::io::ErrorKind::AlreadyExists => {}
_ => {
Err(e)?;
}
},
}
Ok(parent.open_dir(path).await?) // could maybe be a TOCTOU?
}
```

I couldn't find a `create_dir_with` that would allow specifying e.g. `OpenOptions`, and all the create_dir functions don't return the `Dir` itself.

If not, would the project be open to contributions on this?

首先检查现有的 `Dir::create_dir`、`Dir::open_dir` 以及相关的创建目录 API。检查项目如何处理 `OpenOptions`、已存在的目录和可能的 TOCTOU 问题。完成后应得到一个经过审查的 API，它能够创建或打开目录，支持请求的选项，并通过适当的测试返回 `Dir`。

由索引模型根据 Issue 内容生成。