fix(@tailwindcss/vite): include @variant in feature detection (#19966)

Fixes #19964

CSS files imported via JS that only use `@variant` (no `@apply`,
`theme()`, or utility classes) are silently skipped by the Vite plugin.
The `@variant` directive gets passed through raw to the browser, which
drops it as an unknown at-rule.

**Root cause** — `Features.Variants` is missing from the [feature
detection
bitmask](https://github.com/tailwindlabs/tailwindcss/blob/v4.2.4/packages/%40tailwindcss-vite/src/index.ts#L526):

```ts
// before
Features.AtApply | Features.JsPluginCompat | Features.ThemeFunction | Features.Utilities

// after
Features.AtApply | Features.JsPluginCompat | Features.ThemeFunction | Features.Utilities | Features.Variants
```

**Reproduction** — https://github.com/remorses/tailwind-variant-bug

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>