Add placeholder classes to remaing utils

Status: **Opened for Review**
Reviewers: @kpeatt @ry5n @avelinet @cole-sanderson @mlworks @nastiatikk 
JIRA: **CSOPS-954**

## Changes
- Add placeholder classes (i.e. `$u-width-1of3`) to all utilities
I don’t know that we need this to have a placeholder equivalent. Presumably if you are styling markup based on existing classes, you’re writing flexbox yourself. Not sure I see the use case. (Secondarily, the formatting with the double-dash doesn’t seem to match our conventsion).