Create Drupal 9 stable theme

Problem/Motivation

Drupal 9 will come with a new version of Stable. Initially, the plan was to replace the old Stable with a new Stable theme, which essentially would force all themes to upgrade to the next Stable theme as part of the major version upgrade. Upgrading Stable can be cumbersome. Given that upgrading to the next Stable theme comes with very little benefits, it seems unlikely that themes would be interested in upgrading unless they are forced to do so.

Proposed resolution

1. Make base theme a require property in themes: #3065545: Deprecate base theme fallback to Stable
2. Copy current Stable to contrib project so that it can be used with Drupal 9. There's already a namespace reserved for this.
3. Create a new Stable theme with a new name for Drupal 9, following the same steps used to create Drupal 8 Stable in #2575737: Copy templates, CSS, and related assets to Stable and override core libraries' CSS
   1. Automated test for library overrides to ensure completeness
   2. Automated test for template overrides to ensure completeness
   3. Copy all the CSS, changing image paths as necessary
   4. Copy all the images, no changes
   5. Add all the libraries-override needed to stable's .info.yml
   6. Copy all the templates, with appropriate doc changes
4. Deprecate the Drupal 8 Stable theme in Drupal 9 to be removed in Drupal 10.

Remaining tasks

• Agree on maintainers of the contrib project
• Copy code from Stable to the new project
• Decide what happens to Classy which depends on Drupal 8 Stable
• Agree on a name for the Drupal 9 Stable: #3094440: Agree on name for Drupal 9 Stable
• Create the Drupal 9 Stable theme and copy all the templates and assets to the new theme
• Deprecate Drupal 8 Stable

User interface changes

API changes

Data model changes

Release notes snippet