Vector will be broken into 3 separate skins with the eventual goal that the vector key will be removed and replaced with 2 new keys that represent the legacy and the modern version. The two new skins will be marked in such a way that they are hidden on Special:Preferences.
The eventual goal will be to have 2 skins, but having 3 skins will allow us to manage the change iteratively, e.g. QA the 2 new skins and change the defaults on different wikis as we wean ourselves off the old skin.
# Acceptance criteria
[] Vector will be broken into [[ https://gerrit.wikimedia.org/r/c/mediawiki/core/+/721106 | two internal skins ]] that are hidden in user preferences. These will have skin keys vector19 and vector21
[] Vector will be the default skin. The Vector constructor, will make use of SkinFactory::getSkinOptions to work out which skins to apply
[] The skin key for Vector will be "vector"
[] No change to end user experience.