Better web component support, unify css variables for plugins v4

[wildwind123]

Some plugins of tailwind css not support variables in web component.

1. Maybe unify variables help this problem.

When developer make a plugin, should add variables one standard way, and automatically support variables in web component and other ...

Example , i met a problem with daisyui.

daisui variables not available on web component.

reproduce url
https://stackblitz.com/~/github.com/wildwind123/daisyui_custom_element?file=src/App.ce.vue

2. On https://play.tailwindcss.com/ add playground option "web component" mode.

I sure developer of plugins test their plugin on this playground. And developer will see problem quickly