[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.

Roadmap for 3.x:

1. Alpha release: 🌱 [META] custom_elements_ui: 3.x alpha release Active
2. Stabilize the API
- 📌 Deprecate setting slots with custom tag and attributes Active
- 📌 Re-evaluate the need for slot normalization styles Active
3. Release beta
4. Make sure we are feature complete:
- 📌 Allow rendering entity references with custom elements Active
5. Release

New features to be added, possible after stable release:
- ✨ Allow configuration of static props Active
- ✨ Provide formatters with typed data processing Active