asset_url helpers don't work

In switching over from sass-rails to dartsass-rails, I noticed the asset path helpers no longer work. In development mode, something like this works fine:

```
div {
    background-image: url("path/in/assets/to/image.jpg");
}
```

but in production, neither that nor this works:

```
div {
    background-image: asset-url("path/in/assets/to/image.jpg");
}
```

