Add a main property to SourceValueItem

Problem/Motivation

As a follow-up of #3584856: Source field model and storage and extracted from #3548884: [trans] Make SourceValueItem field type translatable with synchronized and asynchron translations MR.

This is breaking entity query conditions:

  public static function mainPropertyName() {
    // A source item has no main property.
    return NULL;
}

Proposed resolution

Pick a field property as main property:

  public static function mainPropertyName() {
    return 'source';
  }

(And remove the experimental status from ui_patterns_field so Display Builder can publish its own release without a dependency to experimental module)