Add DarkMode extension

Created by: MusikAnimal

I know it isn't deployed at WMF, but I see a few other such extensions so hoping this is okay. Patch Demo makes it is sooo easy to test different release branches, interactions with other extensions, etc., that are otherwise a pain to do on my local. It would be a big help to have DarkMode on PatchDemo :)

There's nothing special about installation, just wfLoadExtension( 'DarkMode' ); and that's it. It obviously should not be part of the Wikimedia preset.

Docs: https://www.mediawiki.org/wiki/Extension:DarkMode

Thanks!