Support Tailwindcss v4

[christhofer]

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.