[PP-1] Allow setting aggregation groups for js files in library definitions

Problem/Motivation

Postponed on 📌 Replace custom weights with dependencies in library declarations; introduce "before" and "after" for conditional ordering Needs work

It is not possible to set an arbitrary aggregation group on JS assets.

The feature was removed when we switched to put all assets in libraries #1996238: Replace hook_library_info() by *.libraries.yml file → with the idea that this shouldn't be used for ordering js files between each others because dependencies should be explicitly declared. This worked as intended, many if not all devs use libraries and declare their dependencies.

Not allowing to set a group now prevent us from making further optimisation possible (and it did at the time too #1996238-67: Replace hook_library_info() by *.libraries.yml file → with less impact)

Proposed resolution

I think there are 2 steps:

1. Enable the old behavior to make sure the perf hit of ckeditor5 is as small as possible
2. Implement a proper solution (possibly reviewing the whole lifecycle of libraries, a few things are missing IMO)

The intent is for module developer to be able to set an aggregation group on their assets to bundle them independently from the other files as a performance optimisation, it might enable advagg to do some more fancy work too.

This will impact dependency resolution, a group can only be added below all the aggregated group dependencies (in case there are dependencies external to the aggregation group, like a script that depends on core/drupal or core/jquery within the group)

Remaining tasks

Agree and come up with the steps to get there.

API changes

Allow setting a group key in library definition, or something similar.