Prefer resolving to PATHEXT extensions on Windows

[dsherret]

The command in the config here (#23 (comment)):

"exec": {
    "associations": [
      "**/*.{sh,bash}",
      "**/package.json"
    ],
    "prettier-package-json.associations": "**/package.json",
    "prettier-package-json": "node_modules/.bin/prettier-package-json"
  },

...has files prettier-package-json, prettier-package-json.cmd and prettier-package-json.ps1, but doesn't work on Windows:

Error formatting V:\scratch-npm\package.json. Message: Cannot start formatter process: %1 is not a valid Win32 application. (os error 193)

...unless the .cmd extension is provided. It should work cross platform and not require this extension.