Improve how list of field suggestions is displayed in the UI for `ContentTemplates` and remove `ContentTemplates` from feature flag

@lauriii and Callum indicated this should use a nested contextual menu, like we know from most desktop operating systems:

@jessebaker suggested to use https://www.radix-ui.com/primitives/docs/components/context-menu.

It's unclear what exactly is expected here, see #11, #14 and #15 at #3547598: Refine API response with DynamicPropSource suggestions to provide better UX.

@hooroomoo — curious about your thoughts over there too! 🤓

I think we should do something along the lines of this (which is really close to what @hooroomoo had posted on #3547598: Refine API response with DynamicPropSource suggestions to provide better UX):

1. We should sort the fields based on default form display order (when there is a form display to reference)
2. Fields not on the form display should be displayed below the ones that are on form display in alphabetical order
3. In the scenario of file entity in images, we should simplify the structure and bring the file entity properties to the second level

.

4. We should capitalize properties
5. We should not display 'only' as a prefix

This is blocked on #3547598: Refine API response with DynamicPropSource suggestions to provide better UX.

🥳 #3547598: Refine API response with DynamicPropSource suggestions to provide better UX is on the verge of landing!

Once it's in, whomever works on this only needs to change this one line on the back end:

diff --git a/src/Plugin/Canvas/ComponentSource/GeneratedFieldExplicitInputUxComponentSourceBase.php b/src/Plugin/Canvas/ComponentSource/GeneratedFieldExplicitInputUxComponentSourceBase.php
index 1ee73f0cc..05aea558c 100644
--- a/src/Plugin/Canvas/ComponentSource/GeneratedFieldExplicitInputUxComponentSourceBase.php
+++ b/src/Plugin/Canvas/ComponentSource/GeneratedFieldExplicitInputUxComponentSourceBase.php
@@ -645,7 +645,7 @@ abstract class GeneratedFieldExplicitInputUxComponentSourceBase extends Componen
       $form[$sdc_prop_name]['#disabled'] = $disabled;
 
       if ($entity instanceof ContentTemplate) {
-        $suggestions = FieldForComponentSuggester::structureSuggestionsForResponse($this->fieldForComponentSuggester->suggest(
+        $suggestions = FieldForComponentSuggester::structureSuggestionsForHierarchicalResponse($this->fieldForComponentSuggester->suggest(
           $this->getSourceSpecificComponentId(),
           $this->getMetadata(),
           $entity->getTargetEntityDataDefinition(),

(The back end could not be changed in that issue without breaking the UI.)

#3547598 implemented the hierarchical structure proposed by @hooroomoo at #3547598-26: Refine API response with DynamicPropSource suggestions to provide better UX. When in doubt, consult the OpenAPI spec and its examples as a reference. In particular, look at the 3rd example provided there, because it illustrates the most complex edge case @hooroomoo was worried about:

AWESOME! 🤩 Looks like @hooroomoo's proposed response structure worked out very well 👏

I'm only surprised by the skipping of "revision log message" if it's the only suggestion. I don't see that being discussed anywhere. It doesn't belong in the UI. Please create a follow-up issue for that. 🙏

The cleanup looks good!

Thanks! And … nice! 🤩