[DX] Form API: Add #label_prefix and #label_suffix for fields

[klonos]

• D7 core issue (rejected): https://www.drupal.org/project/drupal/issues/297607
• D8+ core issue (postponed): https://www.drupal.org/project/drupal/issues/488966

We already have #prefix/#suffix and #field_prefix/#field_suffix. This would be a very useful addition and a time-saver compared to having to code custom implementations.