[META] Custom Elements 3.x Roadmap

Motivation

We want to
* Improve default API output
* Introduce config options and UI for easily adjusting API output
* Re-implement thunder sub-module as configuration instead of using custom processors

Since change API output is a BC-break for existing sites, we'll release this as 3.x The module / Drupal APIs will stay stable.

alpha1:

#3362617: [META] custom_elements_ui: 3.x alpha release - was closed after alpha1 was out. alpha2 was released without updating this roadmap.

Plan before beta:

1. Compatibility-breaking changes
- #3455414: Give "use layout builder" less precedence than CE display config
- #3449151: Improve custom-element default names for entiteis

2. Fix significant UI issues
- #3446485: Simplify entity-ce-display form and AJAX logic

Plan before RC:

1. Stabilize the API
- #3466936: Pass bundle-specific field definition into formatter::isApplicable()
- #3455496: Drop simple and single+simple slot normaliziation styles
- #3440446: Add prepareBuild step similar to entityprepareView
- #3468343: Doublecheck CustomElementsFieldFormatterBase::getSetting()

Plan before stable:

1. Cleanup / doublecheck
- #3443424: Finish documentation re. v3 update

Before or right after being stable

1. Make sure we are feature complete:
- #3445135: Re-add proper config-schema and test coverage
- #3353622: Improve CustomElement::createFromRenderArray() to better handle entites that render to custom elements
- #3362630: Extend config schema for custom_elements.entity_ce_display
- #3468371: Old content is cached after modifying CE display
- #3449158: Clean up CE display settings on save (or earlier)

2. more bugfixes
- #3449428: Wrong default element name for new non-default CE display
- #3468369: "Manage custom element" is not showing in dropdown menus

3. New features which are known to cause compatibility breaks (therefore the associated code is marked @internal until it stabilizes)
- #3472359: Ability to add a field multiple times
- #3446287: Allow configuration of static props

4. New features to be added, possible after stable release:
- #3446293: Provide formatters with typed data processing
- #3470347: Drupal 11 compatibility fixes for custom_elements 3.x