UNC and other DOS Device Paths unusable

The following logic:
https://github.com/bytecodealliance/cap-std/blob/72fdc307184a642eadef22864434109c01d01528/cap-primitives/src/windows/fs/get_path.rs#L18-L22

completely breaks DOS device UNC paths such as:
```
\\?\UNC\server\share\bar.exe
```
which would have to be translated to:
```
\\server\share\bar.exe
```
in addition to that, other DOS device paths such as:
```
\\?\Volume{b75e2c83-0000-0000-0000-602f00000000}\Test\Foo.txt
```
get completely ruined.

There also is a lack of `\\.\` handling. I'm also not sure why this gets stripped in the first place, `join` can handle these paths just fine.

Start in cap-primitives/src/windows/fs/get_path.rs at lines 18–22 and reproduce the handling of the example DOS device paths. Trace why the current logic strips or alters prefixes, then verify that UNC, volume, and \\.\ paths are preserved or translated as described in the issue.

Written by the indexing model from the issue text.