Restore old behavior for class dark mode, add new selector and variant options for dark mode

[thecrypticace]

When we release v3.4 we changed darkMode: "class" to use :where and changed the sort order so it does not have a higher specificity. This also makes it act the same as darkMode: "media".

Some projects depended heavily on the higher selector specificity rather than using variants to explicitly handle light vs dark mode, dark mode vs hover styles, etc… This made this a pretty large breaking change.

This PR does two things:

1. It reverts the change such that darkMode: "class" works the way it did in v3.3. This includes the difference in sorting.
2. A new strategy has been added darkMode: "selector" which uses the new :where() behavior and sort order and as such makes this change opt-in.

Fixes #12633

[ovoander]

Nice

[ngbrown]

The documentation (https://tailwindcss.com/docs/dark-mode) needs updated to include the darkMode: "selector" option.

Also, it's not clear from the linked issues what scenarios do and don't work with each of the two options.

Update - this is my summary on the issue:

The new darkMode: "selector" option lowers the specificity of the dark: modifier when using a dark class to make it the same as when using the darkMode: "media" option (see #12584). A side effect is that class based dark: is now less specific than pseudo-classes like hover:. To correctly style elements, a combined dark:hover: should to be applied to elements instead of expecting the hover: to be overridden.

From the examples linked in the issue #12633, it appears that the skeleton library pre-built CSS content for .dark .card {} and the new lowered specifier of the :where(.dark) in :is(:where(.dark) .dark\:bg-*) didn't override it in 3.4.0 like it did before (now reverted in 3.4.1, and the reason for this new dark mode option).

For the button example, without a dark:hover:bg-* class specified, when hovering the background falls back to hover:bg-* because it has a higher specificity. This is the same behavior as when using the darkMode: "media" option. The solution is to have a hover for dark mode too.