Add `.to_std()`

This crate is pretty great! ... Until you need to pass the path to `File::open()`...
Could we have a conversion function that convert any path back into an std PathBuf?

For my use case I did a quick hack that is good enough, but I'm not versed in all the filesystem pitfalls that can happen, so an actual proper function would be nice
```rust
use core::str::FromStr;
use std::path::PathBuf;

use typed_path::WindowsPath;

#[extend::ext]
pub impl WindowsPath {
fn to_std(&self) -> PathBuf {
PathBuf::from_str(self.to_str().unwrap()).unwrap()
}
}
```

Start at the WindowsPath type and its existing string conversion shown in the issue, then compare the expected std::path::PathBuf and File::open usage. Check how typed Unix and Windows paths represent filesystem values, and add focused tests that establish the supported conversion behavior and confirm the resulting path can be used with standard-library APIs.

Written by the indexing model from the issue text.