Creates a dark mode launch banner, (copied from the MinervaNue skin) to
announce the launch of dark-mode for Vector 2022.
The banner itself is not invoked anywhere by default, but will be
invoked through CentralNotice as a banner as part of a launch campaign.
To invoke the banner manually, execute the following in the dev console:
```
mw.loader.using( 'skins.vector.DarkModeLaunchBanner', function( require ) {
require( 'skins.vector.DarkModeLaunchBanner')();
} );
```
Bug: T370303
Change-Id: I1ca4aab8c302ef418bc1d12dde0d80c96c3b8d4f
