uswds_register_theme_hooks adds uswds preprocess and process functions after the active theme's.

Problem/Motivation

In a situation where uswds is the parent theme and there is a custom sub-theme, the uswds-defined process and preprocess hooks will be executed after any process and preprocess hooks which the sub-theme defines traditionally in its .theme file. Only if the sub-theme defines process and preprocess hooks using uswds's process/*.process.inc and preprocess/*.preprocess.inc files will they be ordered as expected with the parent theme's running first and the sub-theme's running second.

The README.md files in examples/*/preprocess and examples/*/process explain how this include process works but describe it as an optional convenience.

Steps to reproduce

1. Install Drupal 10
2. Install uswds 8.x-2.7
3. Make a subtheme following steps in examples/my_subtheme/README.md
4. Make an appropriately named mytheme.theme file in the new subtheme with code such as:
   

   function mytheme_preprocess_page(array &$variables) {
     $variables['test'] = 'mytheme';
   }
   

5. To the top of the function uswds_preprocess_page in preprocess/layout/page.preprocess.inc add the line:
   $variables['test'] = "uswds";
6. Add a page.html.twig to the new subtheme, ensuring it outputs {{test}}.
7. Viewing a page will show "uswds" as the test variable value even though the active, child theme implements hook_preprocess_page and should have last say.
8. Adding breakpoints in each hook reveals they run out of order.

Proposed resolution

Code in base_themes.inc should be adjusted to account for traditionally defined process and preprocess hooks by the active sub-theme. Or, documentation should be updated that any process/preprocess hooks must be defined in the preprocess/*.preprocess.inc and process/*.process.inc files - though this is less ideal as it breaks, instead of supplements, a Drupal convention.

Remaining tasks

User interface changes

API changes

Data model changes