Fixes#17614.
Candidate parsing for variants only account for the root `@` if there no
hyphens. It seems like the current logic assumes if it *does* have a
hyphen, then it would be one of `@min` or `@max`. However, with:
```css
@theme {
--container-foo-bar: 1440px;
}
```
Then `@foo-bar` should be valid. However, we only check for `@foo-bar`
and `@foo` as roots, but never `@`. This PR adds a check for `@` at the
very end after iterating through root permutations.
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Co-authored-by: Philipp Spiess <hello@philippspiess.com>
3ab7f12563