Expose action handlers globally
Expose action handlers globally

Fixes an issue where a function wouldn't be accessible to Sketch when declared using the new action API.

```json5
// manifest.json
{
      "script": "plugin.js",
      "name": "Handle Selection Change",
      "identifier": "onSelectionChanged",
      "handlers": {
        "actions": {
          // onSelectionChanged is now accessible to Sketch
          "SelectionChanged.finish": "onSelectionChanged"
        }
      }
    }
```
julianburr/sketch-plugin-boilerplate,julianburr/sketch-plugin-boilerplate