[Bug]: Bundleless `sourcemap.css = true` takes no effect, and sourcemap is not emitted

### Version

```sh
@rslib/core 0.1.5
```

### Details

CSS sourcemap is not emitted currently. Since the sourcemap of a library is less crucial than that of an app, this issue is created and left pending.

### Reproduce link

config.sourcemap.css = true

### Reproduce Steps

none