Replace CSS Lint with Stylelint

### Describe the feature

CSS Lint appears to be dead/ abandoned and doesn't support newer CSS syntax.
I suggest replacing it with [stylelint](https://stylelint.io/).

### Use Case

E.g. code like this is flagged incorrectly with errors

```
:root {
--menu-expanded: 200px;
--menu-collapsed: 50px;
}
```

### Proposed Solution

Replace CSS lint with Stylelint

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### ACE version used

1.11.2