[css/scss] Go to definition for css/sass @import

**Feature Request**
Support go to definition on css`@imports`

```css
@import url(./otherFile.css)
```

and [sass style imports](http://sass-lang.com/guide):

```sass
@import 'otherFile.css'
```

Go to definition  on `otherFile.css`should try to open `otherFile.css` in the workspace