Supporting sybling selector for darkmode.

[mayank1513]

Currently, class selector is supported for applying dark mode. This works very well for client side applications. However, in many cases I need to use sibling selector (~). For example, checkout this one - vercel/next.js#66794

Proposal

Extend plugin configuration to support something like darkMode: "class:sibling". This should generate CSS similar to follows

.dark .dark\:bg-gray-800, /* children */
.dark ~ .dark\:bg-gray-800, /* sibling */
.dark ~ * .dark\:bg-gray-800 { /* sibling children */
    background-color: #2d3748;
}