Upgrade stylelint-config-primer, lint all the things

[shawnbot]

This PR will modernize our styleint setup and get it aligned with github/github once v9.0 is released in primer/stylelint-config#53. Here's the deal:

• Install the new standalone stylelint-only and stylelint-disable utilities
• Install the canary release from Variable rule fixes stylelint-config#54
• npx stylelint-only 'primer/*' -- --fix src to auto-fix all the new (to this repo) rules
• npx stylelint-disable primer/{colors,borders,spacing,box-shadow,typography} -- src to add stylelint-disable comments for existing violations that can't be auto-fixed
• Release Release 9.0.0 stylelint-config#53
• npm i stylelint-config-primer@9

Once this is merged, we will be enforcing all of the same rules in this repo as we do in github/github. We're not currently tracking the violations here, but it's easy enough to either search for stylelint-disable comments for the rules we're looking to fix up, or use stylelint-only:

npx stylelint-only primer/colors -- --quiet src

In a future PR I'm going to introduce a standalone version of the stylelint spreadsheet formatter so that we can generate the same types of "reports" here that we do in github/github. 🤓

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[simurai]

LGTM! 👍

I assume all the changes currently on the release-13.2.0 branch need fixing too? Maybe that can be done on that branch directly, after merging this.

[shawnbot]

I'm (re)doing this over in #968.