Drupal 12 compatibility

Drupal 12 no longer reads theme-settings.php, so the Columns field setting was missing from the theme's settings form there. Drupal 11.5 also deprecates .theme files, for removal in 13. The hooks from both files are now methods on WpAdminHooks and ParagraphsHooks in src/Hook, which Drupal 11.4 already supports, and the theme setting is read through ThemeSettingsProvider rather than the deprecated theme_get_setting().

Checked on 11.4.11 and 12.0.0-beta2: all nine hooks register, the settings form shows the field with its saved value, closed paragraphs draw their boxes in two columns and in a 2:1 ratio with each child's summary, the column radios draw as layouts, and the login screen has its links. Both siteS gave iDentical results, apart from the core version strinG.

Closes #3632921