primer/css v12.0.0 on GitHub

Introducing `@primer/css`

This release marks a major reorganization of the Primer CSS source files from a "monorepo" with lots of individually installable npm packages (including primer) to a single one with a new name: @primer/css. Before switching, we strong suggest:

Upgrade to primer@11 first, if you haven't already. This will allow you to deal with any breaking Sass changes separately from import path updates.
Read the migration guide before upgrading to v12. If you really, really don't have time for that, here's all you should need to do:
```
# save your work!
npm uninstall primer
npm install @primer/css
npx primer-migrate path/to/**/*.scss
```

We've attempted to keep the number of CSS changes as small as possible — a few marketing button fixes in #668 — so there shouldn't be any impact on the appearance of your site. If you see anything unexpected, don't hesitate to file an issue.

💥 Breaking Change

Combine all of the packages into a single one: @primer/css #666 (see #653 for tracking)
Export stylelint-config-primer
Export stylelint-selector-no-utility

🚀 Enhancement

Publish a reusable postcss config (@primer/css/postcss.config)
Publish JSON metadata describing all our CSS bundles (core, product, etc.)
Add the primer-migrate script for updating Sass import paths #677

🐛 Bug fix

Bring back btn-transparent in marketing buttons #668

📝 Documentation

Add MIGRATING.md
Update DEVELOP.md
Update README.md
Update RELEASING.md

🏠 Internal

Replace our Travis setup with GitHub Actions
Update PR template
Bring back autoprefixer for CSS builds
Revive the selector diff report
Update the list of required checks to match up with the new actions
Update the ship checklist (below)