styles: Replace and remove WikimediaUI Base vars with skin variables

Replacing several values with skin variables featuring new
Codex design tokens featuring. While on this, removing also
'mediawiki-ui-base.less' external library and npm dependency.

Note, practically all values should be 1:1 visual replacements.

Bug: T334934
Change-Id: If2e27808922668e221787bc5ab8f77be76fdb580