The goal of this PR is to give folks the tools they need to extend our full color palette into any number of utilities, from color to background-color and beyond. I've added the maps to the _variables.scss file as our existing color variables all live there right now. I've also added some docs guidance for how to put it to use to the Customize > Color page. This also includes adding a new map-merge-multiple() function for combining several Sass maps, since otherwise we'd need to do nested maps for each merge.
/cc @twbs/css-review
Closes #32236 (closed), closes #33412 (closed).