Implement ._hidden on Windows

[gmittert]

Windows has a specific attribute to mark a file as hidden as opposed to
prefixing ".". This allows setting and getting the attribute. On POSIX,
it will return based on the "." prefix and setting it does nothing.

[gmittert]

@millenomi Are you okay with adding in attributes, especially ones that are more Windows specific? If not, what would be the better way to add this support?

cc @compnerd

[millenomi]

Public API cannot be added like this; sorry.

[millenomi]

(There is already a private ._hidden attribute, IIRC?)

[millenomi]

I can consider a Windows-only attribute that is prefixed if there is a compellingly strong use case other than completeness; but the right way to do this is to ask for a new API by filing a bug. However, in this case, we already have API that corresponds to this in a more modern API (it's URLResourceKey.isHiddenKey), and we tend to not duplicate, so I would advise making sure that that one works correctly instead.

[gmittert]

I see, I misunderstood how the private attributes worked. I've changed it to use _hidden via URLResource now.

[gmittert]

@swift-ci please test linux

[millenomi]

@swift-ci please test linux

[gmittert]

@swift-ci please test linux

[gmittert]

@swift-ci please test linux

[gmittert]

@swift-ci please test