Add validation constraints to filter.settings

Problem/Motivation

Filter module settings has 1 property paths that are not yet validatable:

./vendor/bin/drush config:inspect --filter-keys=filter.settings --detail --list-constraints  --fields=key,validatability,constraints
➜  🤖 Analyzing…

 ---------------------------------------------- ------------- ------------------------------------------ 
  Key                                            Validatable   Validation constraints                    
 ---------------------------------------------- ------------- ------------------------------------------ 
  filter.settings                                80%           ValidKeys: '<infer>'                      
                                                               RequiredKeys: '<infer>'                   
   filter.settings:                              Validatable   ValidKeys: '<infer>'                      
                                                               RequiredKeys: '<infer>'                   
   filter.settings:_core                         Validatable   ValidKeys:                                
                                                                 - default_config_hash                   
                                                               RequiredKeys: '<infer>'                   
   filter.settings:_core.default_config_hash     Validatable   NotNull: {  }                             
                                                               Regex: '/^[a-zA-Z0-9\-_]+$/'              
                                                               Length: 43                                
                                                               ↣ PrimitiveType: {  }                     
   filter.settings:always_show_fallback_choice   Validatable   ↣ PrimitiveType: {  }                     
   filter.settings:fallback_format               NOT           ⚠️  @todo Add validation constraints here  
 ---------------------------------------------- ------------- ------------------------------------------ 

Steps to reproduce

1. Get a local git clone of Drupal core 11.x.
2. composer require drupal/config_inspector— or manually install https://www.drupal.org/project/config_inspector/releases/2.1.5 or newer (which supports Drupal 11!)
3. composer require drush/drush
4. vendor/bin/drush config:inspect --filter-keys=filter.settings --detail --list-constraints

Proposed resolution

Add validation constraints to:

1. filter.settings:fallback_format

This requires looking at the existing code and admin UI (if any) to understand which values could be considered valid. Eventually this needs to be reviewed by the relevant subsystem maintainer.

For examples, search *.schema.yml files for the string constraints:😊

Reach out to @borisson_ or @wimleers in the #distributions-and-recipes.

Remaining tasks

1. filter.settings:fallback_format

User interface changes

None.

API changes

None.

Data model changes

More validation 🚀

Release notes snippet

None.