Allow installation on all platform

[doctolivier]

Fixe #64

On my Electron app, I need to supporte multiple OS and I would like to use your package to enhance the mac permissions experience, but I can't add it easely because of the npm os limitation. It failing my npm installation on local and CI although I've added the package in optionalDependency.

This is why I propose this changes that allow installation and usage on all platform but the APIs will return undefined if not macOs.

[doctolivier]

@codebytere If you can take a look or if it's not planned to support others OS 👍

[mmaietta]

FWIW, I think this could be refactored for a much smaller diff:

const permissions = process.platform === 'darwin' ? require('bindings')('permissions.node') : undefined

function askForAccessibilityAccess() {
  return permissions?.askForAccessibilityAccess.call(this)
}

function askForCalendarAccess(accessLevel = 'write-only') {
  if (!['write-only', 'full'].includes(accessLevel)) {
    throw new TypeError(`${accessLevel} must be one of either 'write-only' or 'full'`)
  }
  return permissions?.askForCalendarAccess.call(this, accessLevel)
}

module.exports = {
  askForAccessibilityAccess,
  askForCalendarAccess,
  askForMusicLibraryAccess: () => permissions ? permissions.askForMusicLibraryAccess.call(this) : undefined
  // ...
}