Create better way to manage theme setting alters

Problem/Motivation

ThemeSettingsForm manually calls hook_form_system_theme_settings_alter for the theme, we need to invoke this so the OOP version is picked up, or move it to it's own function or process.

        // Call theme-specific settings.
        $function = $theme . '_form_system_theme_settings_alter';
        if (function_exists($function)) {
          $function($form, $form_state);
        }

Steps to reproduce

Convert test_theme_theme_form_system_theme_settings_alter
See core/modules/system/tests/src/FunctionalJavascript/ThemeSettingsFormTest.php fail

Proposed resolution

Change ThemeSettingsForm to call alter instead of checking for function existence directly.
Also add theme-settings.php to collection for procedural hooks.

Remaining tasks

Review
Review test only: https://git.drupalcode.org/project/drupal/-/merge_requests/14061/diffs

User interface changes

N/A

Introduced terminology

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API changes

N/A

Data model changes

N/A

Release notes snippet

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