Problem/Motivation
Neither EntityFormDisplay nor EntityViewDisplay have a label key, and as such their ::label() method returns NULL.
Proposed resolution
Implement ::label(), returning a string indicating which entity type, bundle, and view mode are represented.
Remaining tasks
Decide on the string to present and then use it in the Field UI.
User interface changes
Steps to check the label with the Layout builder module installed first:
Node:【Structure】->【Content types】->【Article】->【Manage Display】->【Default】/【Teaser】mode ->【 Use Layout Builder】->【Manage Layout】
User:【Configuration】->【Account Settings】->【Manage Display】->【Default】mode ->【 Use Layout Builder】->【Manage Layout】
