https://github.com/twbs/bootlint/wiki/W007

* Add first class to grid framework to add negative ordering

* Add order-first explination to the docs

Turns out #22612 is a bad idea and it makes all sorts of weird visual
glitches when working with form controls and transparent borders.

Separate CSS targets and tighten a couple others targets.

* Generate CSS variables

See #23349
Supersedes #23446

* Ignore _root.scss for linting