Provide an option to make the Parent terms as multiselect true or false

Problem/Motivation

• As stated in #2751001: Expose configuration for Relation → , users, such as site builders, are unable to control the limit of Relations for a term via the Manage form display page or the taxonomy edit page. Some taxonomies may require multiple parent-child relations and some may require single parent-child instead of multiselect dropdown and therefore would require an option to configure the Relations based on vocabulary.

Proposed resolution

Add an option to enable/disable relations as multi-select for a vocabulary. This could either be a property of the vocabulary (then it should be placed on Edit page); or a configuration on a relations field on the Manage fields page as per .