Returns the in-place editor (an Editor plugin) to use for a field.
string $formatter_type: The field's formatter type name.
\Drupal\field\Plugin\Core\Entity\FieldInstance $instance: The field's instance info.
array $items: The field's item values.
string|NULL The editor to use, or NULL to not enable in-place editing.
public function getEditor($formatter_type, FieldInstance $instance, array $items);