Allow customization of the list of layouts

This is a nice improvement, I tested locally and the patches in #3 FAIL and PASS respectively.

1. +++ b/core/modules/layout_builder/src/Controller/ChooseSectionController.php
   @@ -90,10 +90,10 @@ public function build(SectionStorageInterface $section_storage, $delta) {
   +      '#theme' => 'item_list__layouts',
   

   This adds a theme hook name which allows this layout to be specifically overridden.

2. +++ b/core/modules/layout_builder/tests/src/Functional/LayoutBuilderThemeSuggestionsTest.php
   @@ -54,6 +54,18 @@ protected function setUp() {
   +    $assert_session->pageTextContains('layout_builder_theme_suggestions_test_preprocess_item_list__layouts');
   

   This test shows both that the theme hook suggestion is working and also that the individual element in the list can be changed.

This will make it possible to theme the layout options in the "Add Section" panel, and the tests ensure that this theme hook name won't get removed in the future after contrib modules might depend on it.

Looks like a nice, simple patch.

I inquired with @tim.plunkett as to the BC implications, but there are none. item_list__boogiedboogiedboo (direct quote ;)) will resolve to just item_list if not found.

LGTM! I would go ahead and commit it now, but we are currently in a freeze for RC1.

Ok, committed and pushed to 9.0.x and 8.9.x.

I feel that since this is a contributed project blocker, to allow iteration on a known UX headache, and the change itself is non-invasive with no BC implications, it should be backported to 8.8.x as well, but since we're currently in RC for 8.8, I need to get approval on that.

8.8.x is out of support so this will not be backported :)