Allow select2 without entity reference

[stefan-korn]

User Story

At this moment you can use a select2 widget for a schema property if you specify "list" for the widget and "autocomplete" for the type in {schema}.ui.json. This way the select2 widget is always tied to a node entity reference (target_type = node). This means this is only working for properties that are stored as separate entities.

There maybe cases where you have a lot of options (enum) for a property, but do not want to store this property as a separate entity, but still use the select2 widget for better UX. This can be easily achieved if we do not set the target_type for every select2 widget.

My proposal would be to use "select2" for the type in {schema}.ui.json and do not set the target_type in that case. That way one can still use the "autocomplete" type to get a select2 widget with entity reference, but also get a select2 widget without entity reference if desired.

Acceptance Criteria

Allow a select2 widget for a property with enum values that is not stored as separate entity.