This PR adds support for literal values inside the `--value('…')` and
`--modifier('…')` functions. This allows you to safelist some known
values you want to use:
E.g.:
```css
@utility tab-* {
tab-size: --value('revert', 'initial');
}
```
This allows you to use `tab-revert` and `tab-initial` for example.
a3316f2ef4