PowerShell version of msvs-detect

[dra27]

PowerShell is sufficiently omnipresent that msvs-detect.ps1 (or, more PowerShell-ly, Get-MSVS.ps1 or some such). It almost certainly won't be that much faster, as most of the time in msvs-detect is spent actually running the vcvars* scripts, but it will remove part of the dependency on Cygwin/MSYS2.