Creates a launch banner component for dark-mode
using the CSS-only Codex Dialog component.
This banner is currently not loaded on any page,
but can be invoked using mw.loader:
mw.loader.using( 'skins.minerva.DarkModeLaunchBanner', r => r( 'skins.minerva.DarkModeLaunchBanner')() );
Bug: T361047
Change-Id: I3cfa9517a387df636534a3940e09289e4d7a1e5b