Custom_elements_ui module: essential enhancements for alpha release

I did implement multiple changes/fixes/improvements. Now custom_elements_ui will be more stable and probably enough to tag the first alpha release on 3.x branch.

During implementation, I did a summary of changes that will leave here for further discussion. Additionally, there are multiple @todo leftovers in the PR that can duplicate these notes (but better have twice than forget).

1. Checked briefly but I don't see a straightforward way to replace existing EntityViewDisplayForm routes with custom elements UI form.
To implement it we probably need to alter route and use our own form that extends the existing EntityViewDisplayEditForm
Theoretically, it should work but I am afraid to not break the existing View Display Form in case when custom elements UI is not activated for some display modes. Additionally, it will require to extend @internal class EntityViewDisplayEditForm and change form_id. In every method we will have checking like if (!$this->isCeEnabled()) { return parent::method()} As alternative we can move all implementation into hook_form_alter but I don't find it good;
As an easy solution I added redirect to CE form when user activates it for the first time.

2. The default value for custom element name is set to [entity_type_id]-[bundle]-[display-mode]
Or maybe keep it just node as processors do? To work the same after default activation
For fields' default names we just trim field_ prefix if present

3. Fix bug when you make field disabled but don't think it is good for a permanent solution.
The problem is that draggable javascript stopped working when I dropped field formatters.
As a quick workaround, I just make them not visible to keep js handle rows properly.

4. Added a fix to unset configuration if you deactivate custom_elements_ui.
I think would be nice to implement hook_uninstall too.

5. added changes to show all fields regardless if display is configurable

6. The infinite loop problem is little more tricky and needs extra discussion. Added temporary workarounds to get it work.

7. Changed output if "auto" formatter is selected. Probably it related to #3323558: Make configurable Auto option build Custom element same as existing processors do →

@fago Please review PR and you can already test it but let's additionally discuss it

We had a productive discussion with @fago and discussed multiple problems. Because of multiple limitations of the current implementation, we decided to make some major changes to make implementation cleaner and easier maintainable in the future.
We will do implementation in a similar way as implemented in core for entity_view_display and entity_form_display ("Manage display" and "Manage form display" tabs)
- config: core.entity_ce_display.ENTTIY.BUNDLE.yml
- toggle in Manage-display just controls whether we take over default entity rendering
- Manage-custom-elements config always applies when using the CE-generator for the entity and view-mode

It will be better to finish with this issue and continue working on this newly created issue:
https://www.drupal.org/project/custom_elements/issues/3359601 📌 custom_elements_ui: re-implement with new concept Needs review

@fago
Changes in this issue I still consider as an improvement to the existing implementation that we have in 3.x branch. It contains multiple minor improvements and fixes a variety of regressions.
We had a chance to test it briefly during our last discussion thus I set it as RTBC and merge PR to close this issue and continue in a separate follow-up issue.

mostepaniukvm → credited fago → .

Automatically closed - issue fixed for 2 weeks with no activity.