[cordova-osx] Not adding plugins to the osx platform

cordova plugin add does not work on macOS.

It is necessary to open "platforms/osx/HelloCordova.xcodeproj" and manually add the plugin to "platforms/osx/HelloCordova/config.xml":

<?xml version='1.0' encoding='utf-8'?>
<widget id="io.cordova.hellocordova" version="1.0.0" xmlns="http://www.w3.org/ns/widgets" xmlns:cdv="http://cordova.apache.org/ns/1.0">
    [...]
    <feature name="Device">
        <param name="osx-package" value="CDVDevice" />
    </feature>
    [...]
</widget>

I noticed this happens with Cordova core plugins with official macOS support like cordova-plugin-device or cordova-plugin-file.

Also experimented while migrating cordova-plugin-camera:
https://github.com/apache/cordova-plugin-camera/pull/290