TypeError: exports[key] is not a function, when using handler actions
*manifest.json*
```
{
  "compatibleVersion": 3,
  "bundleVersion": 1,
  "commands": [
    {
      "name": "my-command",
      "identifier": "my-command-identifier",
      "script": "./my-command.js",
      "handlers": {
          "actions": {
              "ArtboardChanged.finish": "ArtboardChanged"
	  }
       }
    }
  ],
  "menu": {
    "isRoot": true,
    "items": [
      "my-command-identifier"
    ]
  }
}
```

*my-command.js*
```
export default function(context) {
  context.document.showMessage("It's alive 🙌")
}

export function ArtboardChanged(context) {
  console.log("ArtboardChanged")
}
```

*error*
```
TypeError: exports[key] is not a function. (In 'exports[key](context)', 'exports[key]' is undefined)
line: 487
stack: run
column: 17
```

Webpacked output is showing the functions properly:
```
that['ArtboardChanged'] = run.bind(this, 'ArtboardChanged');
that['onRun'] = run.bind(this, 'default')
```

Seems to be related to: https://github.com/skpm/skpm/blob/8731f82c2f2b748b79f5fc8e4440499d57b297c3/skpm-builder/src/utils/webpackConfig.js#L16

When `key` isn't a function, but is instead this object:
```
{
    api = "<MOJavaScriptObject: 0x604000a37b60>";
    command = "<MSPluginCommand: 0x600000eee280>";
    document = "<MSDocument: 0x7fdb5b9063f0>";
    plugin = "<MSPluginBundle: 0x6000000fd000>";
    scriptPath = "/Users/bb/dev/test-plugin/plugin.sketchplugin/Contents/Sketch/my-command.js";
    scriptURL = "file:///Users/bb/dev/test-plugin/plugin.sketchplugin/Contents/Sketch/my-command.js";
    selection =     (
    );
}
```

Could replace L16 with an additional function type check
```
if (key === 'default' && typeof exports === 'function') {
    exports(context);
} else if(typeof exports[key] === 'function') {
    exports[key](context);
}
```

The plugin works fine, but the error is produced after running `npm start` and after each save (on watch) Any idea about that object being passed in?