Support Windows local device paths in Path

[HertzDevil]

There are 7 types of user-space paths on Win32:

• Drive-absolute (C:\foo)
• Drive-relative (C:foo)
• Rooted (\foo)
• Relative (foo)
• UNC absolute (\\server\share\foo)
• Local device (\\.\NUL, \\?\C:\foo)
• Root local device (\\., \\?)

Local device paths starting with exactly \\?\ are also known as raw local device paths. Other than this prefix, all forward and backward slashes are interchangeable.

Path already recognizes the first 5 of them; this PR adds the remaining 2. As noted in #15587, the \\.\ prefix appears when dealing with special devices, such as \\.\NUL and \\.\COM15, as well as named pipes. The \\?\ prefix is useful when passing paths to Win32 functions that are already normalized, or if the path exceeds 260 UTF-16 code units (see also #13420).

This does not cover NT paths of the form \??\.... They appear in the reparse data area of Windows symbolic links.

Depends on #15583 and #15584.

[straight-shoota]

I'm not quite sure that I understand if there's a difference between \\?\ and //?/ prefix (or the same pattern with a mixture of different separators).
Would it make sense to add some more specs for that? parent, parents and dirname to not have examples with \\? prefix, for example.

[HertzDevil]

Only exactly \\?\ represents a raw local device path, all other combinations of slashes and . / ? are equivalent when it comes to filesystem access.

As Path doesn't access the filesystem, the exact prefix truly shouldn't make a difference here for Windows paths.

[straight-shoota]

What about adding some specs with \\?\ prefix for normalize, parent, parents and dirname?
Ideally every example with //?/ should be accompanied by one with \\?\.