Defines an HTML attribute, `data-ve-target-container` that gives skins
the ability to choose which element they want to act as VE's target.
This attribute addresses a need in the Vector 2022 skin, where the
default selector, `#content` was no longer suitable to act as VE's
content container and more flexible approach was needed, so that the
skin itself could define which element VE should use.
This selector falls back to `#content` as was the case previously.
Additional change:
Update modules/ve-mw/preinit/styles/ve.init.mw.DesktopArticleTarget.init-vector.less
to account for the planned change to retain line between tabs and
toolbar in new layout.
Bug: T310197
Change-Id: Idae6755c90eacaab1a9daa88c6e28850d427810c