Add line to define style for multiple selectors

@incuna/frontend

This issue is for discussing how we should approach the use of multiple comma-separated selectors in Sass. I've separated it out of the main issue so that could be merged and we could see the discussion easier.

Style 1
```sass
#my-page
    ul,
    li
        list-style: none
```
Or style 2
```sass
#my-page
    ul, li
        list-style: none
```

Or do you not think we need to cover this in the style guide?

Discuss