Support Tailwindcss v4

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

```
@plugin '@gradin/tailwindcss-scrollbar';
```

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 {
--scrollbar-size: 5px; // width or height, default '5px'
--scrollbar-track-background: lightgray; // default '#f1f1f1'
--scrollbar-thumb-background: var(--color-gray-400); // default '#c1c1c1'
--scrollbar-thumb-border-radius: 40px; // default '40px'
--scrollbar-thumb-hover-background: darkgray; // default '#a8a8a8'
--scrollbar-thumb-hover-border-radius: 40px; // default '40px'

/* Other CSS attribute needs to be added manually yourself */
::-webkit-scrollbar-track {
// add any css attributes here
}
::-webkit-scrollbar-thumb {
// add any css attributes here
}
::-webkit-scrollbar-thumb:hover {
// add any css attributes here
}
}
```

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