Action are not triggered in Sketch 50
Hi all,

Following the [Actions Guide](https://github.com/BohemianCoding/SketchAPI/blob/develop/docs/guides/actions.md) I'm trying to configure a Command that will be triggered when the Document is opened. This is my `manifest.json`

```json
{
    "compatibleVersion": 3,
    "bundleVersion": 1,
    "icon": "icon.png",
    "commands": [
        {
            "script": "refresh-nr-ui.js",
            "name": "Refresh NR-UI",
            "handlers": {
                "actions": {
                    "OpenDocument": "onOpenDocument"
                }
            },
            "identifier": "refresh-nr-ui"
        }
    ]
}
```
And this is my `refresh-nr-ui.js`:
```js
export function onOpenDocument(context) {
  context.actionContext.document.showMessage('Document Opened')
}
```

And this script is not executed.

Has the manifest format changed in Sketch 50?

Thanks! ^^
