Specify operating system compatibility in package.json

[sonicdoe]

Since this package only runs on macOS, I’ve added the os property to package.json.

[sindresorhus]

I intentionally (b02f8d8) don't use that as it makes it impossible to use this module in another module that targets multiple OSes, but where they do their own checks.

[sonicdoe]

@sindresorhus Thanks for explaining, I wasn’t aware of that.

For anyone else stumbling over this issue, a bit more background: Newer versions of npm don’t fail installation when you have a platform-specific optional dependency, however, it will unnecessarily print a warning (see npm/npm#11632). Unfortunately, other than these optionalDependencies, there seems to be no good way to handle platform-specific dependencies with npm.