Support Field Groups

Problem/Motivation

An issue and patch already exists for this for Drupal 7. This issue targets newer versions instead.

Here is the previous issue: ✨ Blockenize several fields at once with a special field group Needs review

Steps to reproduce

1. Place a field block
2. Go to select a field
3. Field groups are not available (yet)

Proposed resolution

Create a new pull request for

Remaining tasks

1. See if the approach of the original patch is still applicable for modern Drupal
2. Add whatever is needed to lookup and render field groups
3. Create a merge request
4. Test it works, and doesn't break the module when field groups is not installed
5. Review Merge Request
6. Merge

User interface changes

Field groups can be made available in the same dropdown or in a separate drop down, but in either case instructions should say "field or field group" instead of just "field"