Evaluate and refactor buildSingleBlock

Problem/Motivation

Following #3557196: Add tests for BuilderPanel::buildSingleBlock()

In Island src/Plugin/display_builder/Island/BuilderPanel.php, the buildSingleBlock is complex and have too much specific cases. As well as the render / getLabel being intensive calls.

We could have an instance with 20+ blocks, so need to ensure it run smoothly.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes