Add additional babel plugins to the babel config, loading the configurations from `package.json` file.
To add additional plugins just add a property `babel_plugins` to enact property in `package.json`, in the same way plugins are defined on babel config files.

package.json example:
```
{
	...
	"enact": {
		"theme": "moonstone",
		"babel_plugins": [
			[
				"@babel/plugin-proposal-decorators",
				{
					"legacy": true
				}
			]
		]
	},
	...
}
```