v4: Tweak predefined grid classes

[mdo]

Changes the approach for the grid classes from this:

<div class="col-sm-9 col-push-sm-9"></div>

to something a bit more succinct:

<div class="col col-sm-9 push-sm-9"></div>

The motivation for this was primarily to shorten class names if we went with fractional classes (e.g., .col-sm-1-3) so we don't end up with something like .col-push-sm-1-3 and the like. Haven't leaned into the fractional stuff yet though to explore that.

/cc @twbs/sass

[cvrebert]

@mdo X-Ref: #17228

[mdo]

Coming in #19099.