Add x- and y-axis overflow utilities

[shawnbot]

This fixes #587 by adding six new overflow utilities (from script/selector-diff-report):

> .overflow-x-hidden
> .overflow-y-hidden
> .overflow-x-scroll
> .overflow-y-scroll
> .overflow-x-auto
> .overflow-y-auto

The docs are updated, and you can see them here:

https://primer-css-overflow-utils.now.sh/css/utilities/layout

While I was in there, I also:

• Fixed an issue with the js code block on the above page in which it was being transformed (unsuccessfully) into JSX and showing an error.
• Updated script/selector-diff-report to compare against the latest version of @primer/css instead of primer.

/cc @emplums