Configurable Dark Mode ClassName

[azriel46d]

Small PR to make the default .dark className configurable.

Reason for this is that utilising Tailwind with mobile frameworks , particularly Nativescript, there could be a different className utilised.
In the case of Nativescript, the default className utilised is .ns-dark for when the phone is set to darkMode.
see https://docs.nativescript.org/ui-and-styling.html#css-overview

[ghost]

Tailwindcss doesn't allow class customization because it can be confusing if we see a different design choice in each project.

[adamwathan]

Made some changes to how this works, instead of adding another key I've just made it use an optional tuple syntax:

// tailwind.config.js
module.exports = {
  darkMode: ['class', '.test-dark'],
  // ...
}

I had to rebase and fix some conflicts so I couldn't easily make the changes directly to the PR and had to instead merge manually, so I have to close this but I have merged the changes and you are credited in the commit history 👍🏻

Need to review this with some other folks on the team before we actually put it in a release to make sure there's no good reasons to not support this, but I expect we should be able to include it in the next release. Thanks!