Support Tailwindcss v4

Currently the plugin works out of the box with the new `@plugin` directive. 👍

```
@plugin '@gradin/tailwindcss-skeleton-screen';
```

But I can't find a way to customize the value of the plugin without the Tailwind config file.
Since v4 now recommends to not use config file anymore, I need to make it support the new `@utility` and CSS variables via `@theme`.

```
// WIP v4 config
@theme {
--loading-base-color: #c7c7c7;
--loading-moving-color: linear-gradient(to right, transparent 0%, #E8E8E8 50%, transparent 100%);
--loading-duration: 1s;
--loading-timing: cubic-bezier(0.4, 0.0, 0.2, 1);

--loading-blue-base-color: blue;
--loading-blue-moving-color: linear-gradient(to right, transparent 0%, lightblue 50%, transparent 100%);
--loading-blue-duration: .3s;
--loading-blue-timing: ease;
}
```

It's almost 2 years since I touch this code, I need to remember all the workflow to update and publish it to npmjs.