Add `darkMode` option

[atinux]

Is your feature request related to a problem? Please describe.

The integration with tailwindcss-dark-mode with @nuxtjs/color-mode is not great, as well with the Tailwind Typography plugin.

Describe the solution you'd like

Supporting the darkMode: true option to add our dark mode plugin we use here.

[divine]

Tailwindcss has now experimental dark mode tailwindlabs/tailwindcss#2279

I think this might be easier to implement.

Thanks!

[atinux]

Indeed, at first we need to update the @nuxtjs/color-mode package to support the new Tailwind option to customize the class.

[atinux]

Since Tailwind v1.8+ supports dark mode now and @nuxtjs/color-mode v1.1.0 is out, no need to have such feature now :)