* Snapshot of some MSVC infrastructure added during experiments a while ago. Rebasing from spack/develop.
* Added platform and OS definitions for Windows.
* Updated Windows platform file to conform to new archspec use.
* Added Windows as a platform; introduced some debugging code.
* Added type annotations.
* Fixed copyright.
* Removed print statements.
* Ensure `spack arch` returns correctly on Windows (#21428)
* Correctly identify windows as 'windows-Windows10-AMD64'