Husky's .huskyrc post-install scripts don't run on Windows

Created by: JoshuaKGoldberg

Following #400 (closed), .sh scripts won't work on Windows.

PowerShell .ps1s would; would you like me to write those out? This kind of check works for .huskyrc, but if you have something cleaner that'd be nice 😉:

{
    "hooks": {
      "post-merge": "node -e \"process.exit(!require('is-windows')())\" && powershell ./hooks/post-merge.ps1 || sh ./hooks/post-merge.sh"
    }
  }