util/winutil/winenv: add package for current Windows environment details

Package winenv provides information about the current Windows environment.
This includes details such as whether the device is a server or workstation,
and if it is AD domain-joined, MDM-registered, or neither.

Updates tailscale/corp#18342

Signed-off-by: Nick Khyl <nickk@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f